F(x)=7x^2-84

Solution for F(x)=7x^2-84 equation:

(F)=7F^2-84

We move all terms to the left:

(F)-(7F^2-84)=0

We get rid of parentheses

-7F^2+F+84=0

a = -7; b = 1; c = +84;
Δ = b2-4ac
Δ = 12-4·(-7)·84
Δ = 2353
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$F_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$F_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$F_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(1)-\sqrt{2353}}{2*-7}=\frac{-1-\sqrt{2353}}{-14}
$
$F_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(1)+\sqrt{2353}}{2*-7}=\frac{-1+\sqrt{2353}}{-14}
$