3x(x+1)=36

Solution for 3x(x+1)=36 equation:

3x(x+1)=36

We move all terms to the left:

3x(x+1)-(36)=0

We multiply parentheses

3x^2+3x-36=0

a = 3; b = 3; c = -36;
Δ = b2-4ac
Δ = 32-4·3·(-36)
Δ = 441
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{441}=21$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(3)-21}{2*3}=\frac{-24}{6}
=-4
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(3)+21}{2*3}=\frac{18}{6}
=3
$