Which of the following expressions represents a function?
a. x2 y2 = 9
b. {(4, 2), (4, –2), (9, 3), (9, –3)}
c. x = 4
d. 2x y = 5

Definition of a function: a relation that assigns to every x in the domain a unique y value in the range. So, for every input (x value) there can only be one output (y value). That being said we know b cannot be correct because 4 and 9 both have two outputs each. We know it can't be c because the line x=4 is a vertical line at (4,0) which would cause 4 to have infinite outputs. B is definitely a function, because if you divide by 2x you get y=5/2x. If you plug in any value for x it will be a function