8x(x-21)=0

Solution for 8x(x-21)=0 equation:

8x(x-21)=0

We multiply parentheses

8x^2-168x=0

a = 8; b = -168; c = 0;
Δ = b2-4ac
Δ = -1682-4·8·0
Δ = 28224
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{28224}=168$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-168)-168}{2*8}=\frac{0}{16}
=0
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-168)+168}{2*8}=\frac{336}{16}
=21
$