0=21x^2+64x-20

Solution for 0=21x^2+64x-20 equation:

0=21x^2+64x-20

We move all terms to the left:

0-(21x^2+64x-20)=0

We add all the numbers together, and all the variables

-(21x^2+64x-20)=0

We get rid of parentheses

-21x^2-64x+20=0

a = -21; b = -64; c = +20;
Δ = b2-4ac
Δ = -642-4·(-21)·20
Δ = 5776
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{5776}=76$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-64)-76}{2*-21}=\frac{-12}{-42}
=2/7
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-64)+76}{2*-21}=\frac{140}{-42}
=-3+1/3
$