-5x(4-x)=9

Solution for -5x(4-x)=9 equation:

-5x(4-x)=9

We move all terms to the left:

-5x(4-x)-(9)=0

We add all the numbers together, and all the variables

-5x(-1x+4)-9=0

We multiply parentheses

5x^2-20x-9=0

a = 5; b = -20; c = -9;
Δ = b2-4ac
Δ = -202-4·5·(-9)
Δ = 580
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:

$\sqrt{\Delta}=\sqrt{580}=\sqrt{4*145}=\sqrt{4}*\sqrt{145}=2\sqrt{145}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-20)-2\sqrt{145}}{2*5}=\frac{20-2\sqrt{145}}{10}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-20)+2\sqrt{145}}{2*5}=\frac{20+2\sqrt{145}}{10}
$