Which equation is not a function?

y = 3x - 20
x + 2y = 8
y = x^2 - 4
x + y^2 = 16

x + y² = 16

Step-by-step explanation:

x + y² = 16 fails the vertical line test.  If you draw this horizontal parabola and then draw a vertical line through the graph, the line will intersect the parabola in 2 places.  This is a sure-fire way to recognize that x + y² = 16 is not a function.

inverse operation of addition