x(x+15)=25

Solution for x(x+15)=25 equation:

x(x+15)=25

We move all terms to the left:

x(x+15)-(25)=0

We multiply parentheses

x^2+15x-25=0

a = 1; b = 15; c = -25;
Δ = b2-4ac
Δ = 152-4·1·(-25)
Δ = 325
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:

$\sqrt{\Delta}=\sqrt{325}=\sqrt{25*13}=\sqrt{25}*\sqrt{13}=5\sqrt{13}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(15)-5\sqrt{13}}{2*1}=\frac{-15-5\sqrt{13}}{2}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(15)+5\sqrt{13}}{2*1}=\frac{-15+5\sqrt{13}}{2}
$