Hi, i'm not sure if this in the right section so apologies if its not.
$\displaystyle 4500 = -5x^2 + 390x - 30$
Move everything to one side (aka, set to zero!)
$\displaystyle 5x^2 - 390x + 4470 = 0$
Simplify this by dividing by five.
$\displaystyle x^2 - 78x + 894 = 0$
Now, use the quadratic equation.
$\displaystyle x = \frac{78 \pm \sqrt{78^2 - 4(894)(1)}}{2}$
$\displaystyle x = \frac{78 \pm \sqrt{6084 - 3576}}{2}$
$\displaystyle x = \frac{78 \pm \sqrt{2508}}{2}$
$\displaystyle x = 39 \pm \sqrt{627}$