x(5x-20)-3(5x-20)=0

Solution for x(5x-20)-3(5x-20)=0 equation:

x(5x-20)-3(5x-20)=0

We multiply parentheses

5x^2-20x-15x+60=0

We add all the numbers together, and all the variables

5x^2-35x+60=0

a = 5; b = -35; c = +60;
Δ = b2-4ac
Δ = -352-4·5·60
Δ = 25
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{25}=5$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-35)-5}{2*5}=\frac{30}{10}
=3
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-35)+5}{2*5}=\frac{40}{10}
=4
$