x*(5x-15)=0

Solution for x*(5x-15)=0 equation:

x(5x-15)=0

We multiply parentheses

5x^2-15x=0

a = 5; b = -15; c = 0;
Δ = b2-4ac
Δ = -152-4·5·0
Δ = 225
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{225}=15$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-15)-15}{2*5}=\frac{0}{10}
=0
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-15)+15}{2*5}=\frac{30}{10}
=3
$