-4y^2+25-20y=0

Solution for -4y^2+25-20y=0 equation:

-4y^2+25-20y=0

a = -4; b = -20; c = +25;
Δ = b2-4ac
Δ = -202-4·(-4)·25
Δ = 800
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:

$\sqrt{\Delta}=\sqrt{800}=\sqrt{400*2}=\sqrt{400}*\sqrt{2}=20\sqrt{2}$
$y_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-20)-20\sqrt{2}}{2*-4}=\frac{20-20\sqrt{2}}{-8}
$
$y_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-20)+20\sqrt{2}}{2*-4}=\frac{20+20\sqrt{2}}{-8}
$