-18=2x^2+15x

Solution for -18=2x^2+15x equation:

-18=2x^2+15x

We move all terms to the left:

-18-(2x^2+15x)=0

We get rid of parentheses

-2x^2-15x-18=0

a = -2; b = -15; c = -18;
Δ = b2-4ac
Δ = -152-4·(-2)·(-18)
Δ = 81
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{81}=9$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-15)-9}{2*-2}=\frac{6}{-4}
=-1+1/2
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-15)+9}{2*-2}=\frac{24}{-4}
=-6
$