1x(2x+21)=90

Solution for 1x(2x+21)=90 equation:

1x(2x+21)=90

We move all terms to the left:

1x(2x+21)-(90)=0

We multiply parentheses

2x^2+21x-90=0

a = 2; b = 21; c = -90;
Δ = b2-4ac
Δ = 212-4·2·(-90)
Δ = 1161
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:

$\sqrt{\Delta}=\sqrt{1161}=\sqrt{9*129}=\sqrt{9}*\sqrt{129}=3\sqrt{129}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(21)-3\sqrt{129}}{2*2}=\frac{-21-3\sqrt{129}}{4}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(21)+3\sqrt{129}}{2*2}=\frac{-21+3\sqrt{129}}{4}
$