If you write it in the form you may find it's easier to see what's going on.
What you do is see whether you can get it in the form where is on its own on one side and there's a complicated manipulation of that you can do to get to .
So you have:
Subtract 4 from both sides:
Divide both sides by 3
which can be written
and it is clear that is a function of .
The next one's trickier:
Multiply by :
The only reason that's not a function is because it's not defined at , and so is not one-to-one. There's no image of 2.
Can you do the third one now?