Which equation is a function of x? 20 ! quick !

a) x = 5 the 2 next to the
b) x = y2 + 9 variable makes it squared
c) x2 = y
d) x2 = y2 + 16

A function of x means that you get an answer if you plug in a certain value for x.

A is not a function, because it specifies that x=5
B could be a function, but you'd have to rewrite it in a form y =
C is a nice function of x. Just swap it: y=x2
D is a circle. You can't rewrite it as a clean function of x. 

So I would choose C.