X(y+6)=91

Solution for X(y+6)=91 equation:

X(X+6)=91

We move all terms to the left:

X(X+6)-(91)=0

We multiply parentheses

X^2+6X-91=0

a = 1; b = 6; c = -91;
Δ = b2-4ac
Δ = 62-4·1·(-91)
Δ = 400
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$X_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$X_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{400}=20$
$X_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(6)-20}{2*1}=\frac{-26}{2}
=-13
$
$X_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(6)+20}{2*1}=\frac{14}{2}
=7
$