Question

Which equation is a function of x?
x = 5
x = y squared + 9
x squared = y
x squared = y squared + 16

Answer 1

x^2 = y (i.e. y = x^2).

Reason: solving for y gives a single value y = x^2 for each x. The others give either no unique y or two y-values for some x (x = 5 is a vertical line; x = y^2 + 9 and x^2 = y^2 + 16 give y = ±√(…) ).