(2x+1)(x-3)=15

Solution for (2x+1)(x-3)=15 equation:

(2x+1)(x-3)=15

We move all terms to the left:

(2x+1)(x-3)-(15)=0

We multiply parentheses ..

(+2x^2-6x+x-3)-15=0

We get rid of parentheses

2x^2-6x+x-3-15=0

We add all the numbers together, and all the variables

2x^2-5x-18=0

a = 2; b = -5; c = -18;
Δ = b2-4ac
Δ = -52-4·2·(-18)
Δ = 169
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{169}=13$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-5)-13}{2*2}=\frac{-8}{4}
=-2
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-5)+13}{2*2}=\frac{18}{4}
=4+1/2
$