3x(x+10)=90

Solution for 3x(x+10)=90 equation:

3x(x+10)=90

We move all terms to the left:

3x(x+10)-(90)=0

We multiply parentheses

3x^2+30x-90=0

a = 3; b = 30; c = -90;
Δ = b2-4ac
Δ = 302-4·3·(-90)
Δ = 1980
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:

$\sqrt{\Delta}=\sqrt{1980}=\sqrt{36*55}=\sqrt{36}*\sqrt{55}=6\sqrt{55}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(30)-6\sqrt{55}}{2*3}=\frac{-30-6\sqrt{55}}{6}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(30)+6\sqrt{55}}{2*3}=\frac{-30+6\sqrt{55}}{6}
$