300=(x+15)(10+x)

Solution for 300=(x+15)(10+x) equation:

300=(x+15)(10+x)

We move all terms to the left:

300-((x+15)(10+x))=0

We add all the numbers together, and all the variables

-((x+15)(x+10))+300=0

We multiply parentheses ..

-((+x^2+10x+15x+150))+300=0


We calculate terms in parentheses: -((+x^2+10x+15x+150)), so:

(+x^2+10x+15x+150)

We get rid of parentheses

x^2+10x+15x+150

We add all the numbers together, and all the variables

x^2+25x+150

Back to the equation:

-(x^2+25x+150)


We get rid of parentheses

-x^2-25x-150+300=0

We add all the numbers together, and all the variables

-1x^2-25x+150=0

a = -1; b = -25; c = +150;
Δ = b2-4ac
Δ = -252-4·(-1)·150
Δ = 1225
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{1225}=35$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-25)-35}{2*-1}=\frac{-10}{-2}
=+5
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-25)+35}{2*-1}=\frac{60}{-2}
=-30
$