A=-x^2-10x+300

Solution for A=-x^2-10x+300 equation:

=-A^2-10A+300

We move all terms to the left:

-(-A^2-10A+300)=0

We get rid of parentheses

A^2+10A-300=0

a = 1; b = 10; c = -300;
Δ = b2-4ac
Δ = 102-4·1·(-300)
Δ = 1300
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$A_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$A_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

The end solution:

$\sqrt{\Delta}=\sqrt{1300}=\sqrt{100*13}=\sqrt{100}*\sqrt{13}=10\sqrt{13}$
$A_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(10)-10\sqrt{13}}{2*1}=\frac{-10-10\sqrt{13}}{2}
$
$A_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(10)+10\sqrt{13}}{2*1}=\frac{-10+10\sqrt{13}}{2}
$