90=3x^2+18

Solution for 90=3x^2+18 equation:

90=3x^2+18

We move all terms to the left:

90-(3x^2+18)=0

We get rid of parentheses

-3x^2-18+90=0

We add all the numbers together, and all the variables

-3x^2+72=0

a = -3; b = 0; c = +72;
Δ = b2-4ac
Δ = 02-4·(-3)·72
Δ = 864
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:

$\sqrt{\Delta}=\sqrt{864}=\sqrt{144*6}=\sqrt{144}*\sqrt{6}=12\sqrt{6}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(0)-12\sqrt{6}}{2*-3}=\frac{0-12\sqrt{6}}{-6}
=-\frac{12\sqrt{6}}{-6}
=-\frac{2\sqrt{6}}{-1}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0)+12\sqrt{6}}{2*-3}=\frac{0+12\sqrt{6}}{-6}
=\frac{12\sqrt{6}}{-6}
=\frac{2\sqrt{6}}{-1}
$