20=-3x^2+75

Solution for 20=-3x^2+75 equation:

20=-3x^2+75

We move all terms to the left:

20-(-3x^2+75)=0

We get rid of parentheses

3x^2-75+20=0

We add all the numbers together, and all the variables

3x^2-55=0

a = 3; b = 0; c = -55;
Δ = b2-4ac
Δ = 02-4·3·(-55)
Δ = 660
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:

$\sqrt{\Delta}=\sqrt{660}=\sqrt{4*165}=\sqrt{4}*\sqrt{165}=2\sqrt{165}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(0)-2\sqrt{165}}{2*3}=\frac{0-2\sqrt{165}}{6}
=-\frac{2\sqrt{165}}{6}
=-\frac{\sqrt{165}}{3}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0)+2\sqrt{165}}{2*3}=\frac{0+2\sqrt{165}}{6}
=\frac{2\sqrt{165}}{6}
=\frac{\sqrt{165}}{3}
$