Pre-Calculus Hnrs questions?

Question

How do you solve this equation: If the point (a,4) is on the graph of f(x)=x^2-3x-6, find a. Please help. Pretty please.

Answer 1

Substituting the coordinates of the point (a, 4) into f(x) = x^2 - 3x - 6 we get:

a^2 - 3a - 6 = 4

a^2 - 3a - 10 = 0

This is just a quadratic equation in a.

a = [3 +/- sqrt(9 + 40)]/2

= (3 +/- sqrt 49)/2

= (3 +/- 7)/2

= 10/2 or -4/2

= 5 or - 2