Y=ax^2-bx-12. find the value of x? when a=1, b=2, y=3 by using general formular

Find the value of x when a=1, b=2, y = 3 by using general formula

After replace number in a, b, y we have

3 = 1ax^2 - 2x - 12

x^2 - 2x - 15 = 0

x^2 - 5x + 3x - 15 = 0

(x^2 - 5x) + (3x - 15) = 0

x * (x - 5) + 3 * (x - 5) = 0

(x - 5) * (x + 3) = 0

x - 5 = 0 or x + 3 = 0

x=5 or x=-3