(7x)(10x+5)=90

Solution for (7x)(10x+5)=90 equation:

(7x)(10x+5)=90

We move all terms to the left:

(7x)(10x+5)-(90)=0

We multiply parentheses

70x^2+35x-90=0

a = 70; b = 35; c = -90;
Δ = b2-4ac
Δ = 352-4·70·(-90)
Δ = 26425
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:

$\sqrt{\Delta}=\sqrt{26425}=\sqrt{25*1057}=\sqrt{25}*\sqrt{1057}=5\sqrt{1057}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(35)-5\sqrt{1057}}{2*70}=\frac{-35-5\sqrt{1057}}{140}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(35)+5\sqrt{1057}}{2*70}=\frac{-35+5\sqrt{1057}}{140}
$