(15-y)(y)=56

Solution for (15-y)(y)=56 equation:

(15-y)(y)=56

We move all terms to the left:

(15-y)(y)-(56)=0

We add all the numbers together, and all the variables

(-1y+15)y-56=0

We multiply parentheses

-1y^2+15y-56=0

a = -1; b = 15; c = -56;
Δ = b2-4ac
Δ = 152-4·(-1)·(-56)
Δ = 1
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$y_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$y_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{1}=1$
$y_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(15)-1}{2*-1}=\frac{-16}{-2}
=+8
$
$y_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(15)+1}{2*-1}=\frac{-14}{-2}
=+7
$