How to prove that the subset sum problem is polynomially reducible to the knapsack problem I want to prove that the subset sum problem is polynomially reducible to the Knapsack problem. Overall I want to show that Knapsack is NP-complete.
There are two parts to showing knapsack is NP-complete

knapsack is in NP
If I show that the subset sum problem is polynomially reducible to the knapsack. 
Then since subset sum problem is known to be NP-hard. Knapsack is NP-Hard

Both 1) and 2) imply that Knapsack is NP-Complete

Is trivial as given any sequence of items we can verify the sum of their value 
and weights in linear time. So the knapsack is in NP. Correct me If I am wrong here.

How do I approach 2)? I am not sure how to phrase the polynomial conversion
from the subset sum problem to the knapsack.
The subset-sum problem is a pair $(S,k)$ and a question whether there exists $S' \subset S$ such that $\sum_{s \in S'} s = k$.
The knapsack problem is a tuple $(w,v,W, V)$ that is weights, values, the knapsack capacity and the target value, and a question can you pick such a set of items $\mathcal{I}$ such that:

$\sum_{i \in \mathcal{I}} w_i \leq W$
$\sum_{i \in \mathcal{I}} v_i \geq V$

Hint: Given an instance $(S,k)$ of subset-sum, is there a way to set the weights and values of our items in the knapsack, so that these inequalities turn into a single equality related somehow to $k$?