(x+1)(x-2)=70

Solution for (x+1)(x-2)=70 equation:

(x+1)(x-2)=70

We move all terms to the left:

(x+1)(x-2)-(70)=0

We multiply parentheses ..

(+x^2-2x+x-2)-70=0

We get rid of parentheses

x^2-2x+x-2-70=0

We add all the numbers together, and all the variables

x^2-1x-72=0

a = 1; b = -1; c = -72;
Δ = b2-4ac
Δ = -12-4·1·(-72)
Δ = 289
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{289}=17$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-1)-17}{2*1}=\frac{-16}{2}
=-8
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-1)+17}{2*1}=\frac{18}{2}
=9
$