15=25u^2

Solution for 15=25u^2 equation:

15=25u^2

We move all terms to the left:

15-(25u^2)=0

a = -25; b = 0; c = +15;
Δ = b2-4ac
Δ = 02-4·(-25)·15
Δ = 1500
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$u_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$u_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

The end solution:

$\sqrt{\Delta}=\sqrt{1500}=\sqrt{100*15}=\sqrt{100}*\sqrt{15}=10\sqrt{15}$
$u_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(0)-10\sqrt{15}}{2*-25}=\frac{0-10\sqrt{15}}{-50}
=-\frac{10\sqrt{15}}{-50}
=-\frac{\sqrt{15}}{-5}
$
$u_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0)+10\sqrt{15}}{2*-25}=\frac{0+10\sqrt{15}}{-50}
=\frac{10\sqrt{15}}{-50}
=\frac{\sqrt{15}}{-5}
$