(2x+1)*(x-1)=90

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

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

We move all terms to the left:

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

We multiply parentheses ..

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

We get rid of parentheses

2x^2-2x+x-1-90=0

We add all the numbers together, and all the variables

2x^2-1x-91=0

a = 2; b = -1; c = -91;
Δ = b2-4ac
Δ = -12-4·2·(-91)
Δ = 729
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{729}=27$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-1)-27}{2*2}=\frac{-26}{4}
=-6+1/2
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-1)+27}{2*2}=\frac{28}{4}
=7
$