15=25u^2+2

Solution for 15=25u^2+2 equation:

15=25u^2+2

We move all terms to the left:

15-(25u^2+2)=0

We get rid of parentheses

-25u^2-2+15=0

We add all the numbers together, and all the variables

-25u^2+13=0

a = -25; b = 0; c = +13;
Δ = b2-4ac
Δ = 02-4·(-25)·13
Δ = 1300
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:

$\sqrt{\Delta}=\sqrt{1300}=\sqrt{100*13}=\sqrt{100}*\sqrt{13}=10\sqrt{13}$
$u_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(0)-10\sqrt{13}}{2*-25}=\frac{0-10\sqrt{13}}{-50}
=-\frac{10\sqrt{13}}{-50}
=-\frac{\sqrt{13}}{-5}
$
$u_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0)+10\sqrt{13}}{2*-25}=\frac{0+10\sqrt{13}}{-50}
=\frac{10\sqrt{13}}{-50}
=\frac{\sqrt{13}}{-5}
$