y^2+15y-81=0

Solution for y^2+15y-81=0 equation:

y^2+15y-81=0

a = 1; b = 15; c = -81;
Δ = b2-4ac
Δ = 152-4·1·(-81)
Δ = 549
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$y_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$y_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

The end solution:

$\sqrt{\Delta}=\sqrt{549}=\sqrt{9*61}=\sqrt{9}*\sqrt{61}=3\sqrt{61}$
$y_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(15)-3\sqrt{61}}{2*1}=\frac{-15-3\sqrt{61}}{2}
$
$y_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(15)+3\sqrt{61}}{2*1}=\frac{-15+3\sqrt{61}}{2}
$