(x+42)(3x)=90

Solution for (x+42)(3x)=90 equation:

(x+42)(3x)=90

We move all terms to the left:

(x+42)(3x)-(90)=0

We multiply parentheses

3x^2+126x-90=0

a = 3; b = 126; c = -90;
Δ = b2-4ac
Δ = 1262-4·3·(-90)
Δ = 16956
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:

$\sqrt{\Delta}=\sqrt{16956}=\sqrt{36*471}=\sqrt{36}*\sqrt{471}=6\sqrt{471}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(126)-6\sqrt{471}}{2*3}=\frac{-126-6\sqrt{471}}{6}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(126)+6\sqrt{471}}{2*3}=\frac{-126+6\sqrt{471}}{6}
$