Decoders and Logic Gates
A hint for the 1st part: You need 8 outputs, this requires two 2-to-4 decoders.  What do you do with the 3rd 2-to-4 decoder?

Re the 2nd part of your question: "what is this gate?"

It's a tri-state buffer.  It's "tri-state" because the output can be one of three states: 0, 1 or Z.  I'll explain Z shortly.

The "side" input is the Enable.  If Enable is asserted (high or 1) then the output follows the main (data) input.  If Enable is not asserted (low or 0) then the output ignores the data input and becomes 'Z', or high impedance.

An output that is at 0 or 1 is actively driving the circuit.  It is effectively tied to power or ground through a switch.  If you were to attach two such outputs together and one was at 0 and the other at 1, then you would be joining power to ground - a short circuit.  This is... bad.

An output that is at Z doesn't do anything at all.  This is useful because you can tie any number of outputs that are at Z together and they won't misbehave.  Even better, you can select any one of the outputs and make it 1 or 0 and that one output will take control, overriding all of the Z's.

This is the basis of how computer buses work - lots of possible drivers, but only one of them enabled (driving 1 or 0) at any one time.