4=-20y2-21y

Solution for 4=-20y2-21y equation:

4=-20y^2-21y

We move all terms to the left:

4-(-20y^2-21y)=0

We get rid of parentheses

20y^2+21y+4=0

a = 20; b = 21; c = +4;
Δ = b2-4ac
Δ = 212-4·20·4
Δ = 121
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$y_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$y_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{121}=11$
$y_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(21)-11}{2*20}=\frac{-32}{40}
=-4/5
$
$y_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(21)+11}{2*20}=\frac{-10}{40}
=-1/4
$