(3u^2)-9u-15=0

Solution for (3u^2)-9u-15=0 equation:

(3u^2)-9u-15=0

a = 3; b = -9; c = -15;
Δ = b2-4ac
Δ = -92-4·3·(-15)
Δ = 261
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:

$\sqrt{\Delta}=\sqrt{261}=\sqrt{9*29}=\sqrt{9}*\sqrt{29}=3\sqrt{29}$
$u_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-9)-3\sqrt{29}}{2*3}=\frac{9-3\sqrt{29}}{6}
$
$u_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-9)+3\sqrt{29}}{2*3}=\frac{9+3\sqrt{29}}{6}
$