x^2+2x=81

Solution for x^2+2x=81 equation:

x^2+2x=81

We move all terms to the left:

x^2+2x-(81)=0

a = 1; b = 2; c = -81;
Δ = b2-4ac
Δ = 22-4·1·(-81)
Δ = 328
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

The end solution:

$\sqrt{\Delta}=\sqrt{328}=\sqrt{4*82}=\sqrt{4}*\sqrt{82}=2\sqrt{82}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(2)-2\sqrt{82}}{2*1}=\frac{-2-2\sqrt{82}}{2}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(2)+2\sqrt{82}}{2*1}=\frac{-2+2\sqrt{82}}{2}
$