Output an upside-down tent Given an integer, output an upside-down tent.

The input determines both the size of the tent (absolute value) and whether the entrance is on the left side (negative numbers) or the right side (positive numbers).

If input = -1:
____
\/_/

If input = -2:
________
\  /   /
 \/___/

If input = -3:
____________
\    /     /
 \  /     /
  \/_____/

If input = 1:
____
\_\/

If input = 2:
________
\   \  /
 \___\/

If input = 3:
____________
\     \    /
 \     \  /
  \_____\/

et cetera


Note that the top of the tent (i.e. the last line) has 2 * abs(input) - 1 underscores.

There can be no leading spaces, such that the first line directly starts with an underscore.

Assume that the input will never be 0.

Your code should be as short as possible.

This challenge is based on a chat mini challenge by Helka Homba, which is allowed to be used in real challenges under the conditions of Calvin's Hobbies Public License.
MATL, 55 53 52 51 bytes
|95cy4*Y&quot;DXytPEt0*yvG0&gt;?P_]!'\/ 'w)95JG|G0&lt;yEq:++&amp;(

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Explanation
Let N denote the input. The code proceeds in three steps.
First, the first line of  4*N underscores is built as a string and is displayed (which removes it from the stack).
Second, the &quot;frame&quot; of the tent is built using the two types of slashes. To do this, a numerical 2D array is created which contains 1 and 2 corresponding to the two types of slashes, and 0 for space.
This is done concatenating four matrices:

An identity matrix of size abs (N);
A matrix of the same size containing 2 in the antidiagonal;
A null matrix of the same size;
A copy of matrix 2.

Concatenating these four matrices vertically gives, using N=3 as an example, the following 4*N × N matrix:
1 0 0
0 1 0
0 0 1
0 0 2
0 2 0
2 0 0
0 0 0
0 0 0
0 0 0
0 0 2
0 2 0
2 0 0

(which, transposed, begins to look like the tent).
We now take care of the sign of the input. If it is positive we simply transpose the above matrix and index into the string '\/ '. Indexing is 1-based and modular, so 1 becomes '\', 2 becomes '/' and 0 becomes ' ', producing the 2D char array
\    /     /
 \  /     / 
  \/     /  

On the other hand, if the input is negative we vertically flip and arithmetically negate the 4*N × N matrix, producing
-2  0  0
 0 -2  0
 0  0 -2
 0  0  0
 0  0  0
 0  0  0
-2  0  0
 0 -2  0
 0  0 -2
 0  0 -1
 0 -1  0
-1  0  0

Index -1 now refers to '/' and -2 to '\'. That is, the two types of slashes have been interchanged, as required. Again transposing and indexing into the string '\/ ' thus gives the reversed tent:
\     \    /
 \     \  / 
  \     \/  

Third, underscores need to be filled into part of the last row of the 2D char array. The horizontal position of this line depends on the sign of the input, and its lenght is abs(N).
Once the corresponding spaces have been replaced by underscores, the result is implicitly displayed, below the initial line of underscores that was already displayed in the first step.