3x(x+28)=90

Solution for 3x(x+28)=90 equation:

3x(x+28)=90

We move all terms to the left:

3x(x+28)-(90)=0

We multiply parentheses

3x^2+84x-90=0

a = 3; b = 84; c = -90;
Δ = b2-4ac
Δ = 842-4·3·(-90)
Δ = 8136
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:

$\sqrt{\Delta}=\sqrt{8136}=\sqrt{36*226}=\sqrt{36}*\sqrt{226}=6\sqrt{226}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(84)-6\sqrt{226}}{2*3}=\frac{-84-6\sqrt{226}}{6}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(84)+6\sqrt{226}}{2*3}=\frac{-84+6\sqrt{226}}{6}
$