390=(2x+6)(2x+15)

Solution for 390=(2x+6)(2x+15) equation:

390=(2x+6)(2x+15)

We move all terms to the left:

390-((2x+6)(2x+15))=0

We multiply parentheses ..

-((+4x^2+30x+12x+90))+390=0


We calculate terms in parentheses: -((+4x^2+30x+12x+90)), so:

(+4x^2+30x+12x+90)

We get rid of parentheses

4x^2+30x+12x+90

We add all the numbers together, and all the variables

4x^2+42x+90

Back to the equation:

-(4x^2+42x+90)


We get rid of parentheses

-4x^2-42x-90+390=0

We add all the numbers together, and all the variables

-4x^2-42x+300=0

a = -4; b = -42; c = +300;
Δ = b2-4ac
Δ = -422-4·(-4)·300
Δ = 6564
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

The end solution:

$\sqrt{\Delta}=\sqrt{6564}=\sqrt{4*1641}=\sqrt{4}*\sqrt{1641}=2\sqrt{1641}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-42)-2\sqrt{1641}}{2*-4}=\frac{42-2\sqrt{1641}}{-8}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-42)+2\sqrt{1641}}{2*-4}=\frac{42+2\sqrt{1641}}{-8}
$