5(-3f-9)f=4

Solution for 5(-3f-9)f=4 equation:

5(-3f-9)f=4

We move all terms to the left:

5(-3f-9)f-(4)=0

We multiply parentheses

-15f^2-45f-4=0

a = -15; b = -45; c = -4;
Δ = b2-4ac
Δ = -452-4·(-15)·(-4)
Δ = 1785
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{-(-45)-\sqrt{1785}}{2*-15}=\frac{45-\sqrt{1785}}{-30}
$
$f_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-45)+\sqrt{1785}}{2*-15}=\frac{45+\sqrt{1785}}{-30}
$