Ah. Here is the theory to help you.
Given a quadratic equation
[tex]ax^{2} + bx + c = 0[/tex]
the quadratic formula gives
[tex]x = \frac{-b \pm \sqrt{4ac}}{2a}[/tex]
So here, with your equation
[tex]y = 4x^{2} - 3x - 9 = 0[/tex]
you have
[tex]x = \frac{-(-3) \pm \sqrt{4 \times 4 \times (-9)}}{2 \times 4}[/tex]
