3x^2-15=72

Solution for 3x^2-15=72 equation:

3x^2-15=72

We move all terms to the left:

3x^2-15-(72)=0

We add all the numbers together, and all the variables

3x^2-87=0

a = 3; b = 0; c = -87;
Δ = b2-4ac
Δ = 02-4·3·(-87)
Δ = 1044
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:

$\sqrt{\Delta}=\sqrt{1044}=\sqrt{36*29}=\sqrt{36}*\sqrt{29}=6\sqrt{29}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(0)-6\sqrt{29}}{2*3}=\frac{0-6\sqrt{29}}{6}
=-\frac{6\sqrt{29}}{6}
=-\sqrt{29}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0)+6\sqrt{29}}{2*3}=\frac{0+6\sqrt{29}}{6}
=\frac{6\sqrt{29}}{6}
=\sqrt{29}
$