240-40x+3x^2=640

Solution for 240-40x+3x^2=640 equation:

240-40x+3x^2=640

We move all terms to the left:

240-40x+3x^2-(640)=0

We add all the numbers together, and all the variables

3x^2-40x-400=0

a = 3; b = -40; c = -400;
Δ = b2-4ac
Δ = -402-4·3·(-400)
Δ = 6400
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{6400}=80$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-40)-80}{2*3}=\frac{-40}{6}
=-6+2/3
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-40)+80}{2*3}=\frac{120}{6}
=20
$