-x^2+2x(84-x)=0

Solution for -x^2+2x(84-x)=0 equation:

-x^2+2x(84-x)=0

We add all the numbers together, and all the variables

-x^2+2x(-1x+84)=0

We add all the numbers together, and all the variables

-1x^2+2x(-1x+84)=0

We multiply parentheses

-1x^2-2x^2+168x=0

We add all the numbers together, and all the variables

-3x^2+168x=0

a = -3; b = 168; c = 0;
Δ = b2-4ac
Δ = 1682-4·(-3)·0
Δ = 28224
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{28224}=168$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(168)-168}{2*-3}=\frac{-336}{-6}
=+56
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(168)+168}{2*-3}=\frac{0}{-6}
=0
$