15x+30x(400-x)=7200

Solution for 15x+30x(400-x)=7200 equation:

15x+30x(400-x)=7200

We move all terms to the left:

15x+30x(400-x)-(7200)=0

We add all the numbers together, and all the variables

15x+30x(-1x+400)-7200=0

We multiply parentheses

-30x^2+15x+12000x-7200=0

We add all the numbers together, and all the variables

-30x^2+12015x-7200=0

a = -30; b = 12015; c = -7200;
Δ = b2-4ac
Δ = 120152-4·(-30)·(-7200)
Δ = 143496225
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:

$\sqrt{\Delta}=\sqrt{143496225}=\sqrt{225*637761}=\sqrt{225}*\sqrt{637761}=15\sqrt{637761}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(12015)-15\sqrt{637761}}{2*-30}=\frac{-12015-15\sqrt{637761}}{-60}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(12015)+15\sqrt{637761}}{2*-30}=\frac{-12015+15\sqrt{637761}}{-60}
$