X(x+8)=66

Solution for X(x+8)=66 equation:

X(X+8)=66

We move all terms to the left:

X(X+8)-(66)=0

We multiply parentheses

X^2+8X-66=0

a = 1; b = 8; c = -66;
Δ = b2-4ac
Δ = 82-4·1·(-66)
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{-(8)-2\sqrt{82}}{2*1}=\frac{-8-2\sqrt{82}}{2}
$
$X_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(8)+2\sqrt{82}}{2*1}=\frac{-8+2\sqrt{82}}{2}
$