algebra problem

Question

if (a,15) is a point on the graph of y=x^2-2x what is a?

Answer 1

If (a, 15) is a point on the graph then (a, 15) is a solution to the equation.

 

15 = a² - 2a

0 = a² - 2a - 15

0 = (a - 5)(a + 3)

 

a ∈ {-3, 5}