F(-5)=3x^2-12

Solution for F(-5)=3x^2-12 equation:

(-5)=3F^2-12

We move all terms to the left:

(-5)-(3F^2-12)=0

We add all the numbers together, and all the variables

-(3F^2-12)-5=0

We get rid of parentheses

-3F^2+12-5=0

We add all the numbers together, and all the variables

-3F^2+7=0

a = -3; b = 0; c = +7;
Δ = b2-4ac
Δ = 02-4·(-3)·7
Δ = 84
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$F_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$F_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

The end solution:

$\sqrt{\Delta}=\sqrt{84}=\sqrt{4*21}=\sqrt{4}*\sqrt{21}=2\sqrt{21}$
$F_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(0)-2\sqrt{21}}{2*-3}=\frac{0-2\sqrt{21}}{-6}
=-\frac{2\sqrt{21}}{-6}
=-\frac{\sqrt{21}}{-3}
$
$F_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0)+2\sqrt{21}}{2*-3}=\frac{0+2\sqrt{21}}{-6}
=\frac{2\sqrt{21}}{-6}
=\frac{\sqrt{21}}{-3}
$