15=-3x(x-5)

Solution for 15=-3x(x-5) equation:

15=-3x(x-5)

We move all terms to the left:

15-(-3x(x-5))=0


We calculate terms in parentheses: -(-3x(x-5)), so:

-3x(x-5)

We multiply parentheses

-3x^2+15x

Back to the equation:

-(-3x^2+15x)


We get rid of parentheses

3x^2-15x+15=0

a = 3; b = -15; c = +15;
Δ = b2-4ac
Δ = -152-4·3·15
Δ = 45
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

The end solution:

$\sqrt{\Delta}=\sqrt{45}=\sqrt{9*5}=\sqrt{9}*\sqrt{5}=3\sqrt{5}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-15)-3\sqrt{5}}{2*3}=\frac{15-3\sqrt{5}}{6}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-15)+3\sqrt{5}}{2*3}=\frac{15+3\sqrt{5}}{6}
$