How to identify whether it's a function, and visualizing how it looks?

Well today I received this question in class:

Identify each type of relation and predict whether it is a function. Then graph each function and use the vertical-line test to determine whether your prediction was correct.

a) y = 5 - 2x

b) y = 2x squared - 3

c) y = y = -3/4 (x + 3) squared + 1

d) x squared + y squared = 25

My question is, how will I know what type of function it is by just looking at the relations, and how would I imagine how each graph of one would look like? (Is there a rule I have to memorize?)

GR.11 Math (I forgot my GR.10 (Crying))