(130-y)*y=4200

Solution for (130-y)*y=4200 equation:

(130-y)*y=4200

We move all terms to the left:

(130-y)*y-(4200)=0

We add all the numbers together, and all the variables

(-1y+130)*y-4200=0

We multiply parentheses

-1y^2+130y-4200=0

a = -1; b = 130; c = -4200;
Δ = b2-4ac
Δ = 1302-4·(-1)·(-4200)
Δ = 100
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$y_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$y_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{100}=10$
$y_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(130)-10}{2*-1}=\frac{-140}{-2}
=+70
$
$y_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(130)+10}{2*-1}=\frac{-120}{-2}
=+60
$