5k^2-20=-21k

Solution for 5k^2-20=-21k equation:

5k^2-20=-21k

We move all terms to the left:

5k^2-20-(-21k)=0

We get rid of parentheses

5k^2+21k-20=0

a = 5; b = 21; c = -20;
Δ = b2-4ac
Δ = 212-4·5·(-20)
Δ = 841
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$k_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$k_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{841}=29$
$k_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(21)-29}{2*5}=\frac{-50}{10}
=-5
$
$k_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(21)+29}{2*5}=\frac{8}{10}
=4/5
$