840=11x+3x*x

Solution for 840=11x+3x*x equation:

840=11x+3x*x

We move all terms to the left:

840-(11x+3x*x)=0

We add all the numbers together, and all the variables

-(+11x+3x*x)+840=0

We get rid of parentheses

-11x-3x*x+840=0

Wy multiply elements

-3x^2-11x+840=0

a = -3; b = -11; c = +840;
Δ = b2-4ac
Δ = -112-4·(-3)·840
Δ = 10201
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{10201}=101$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-11)-101}{2*-3}=\frac{-90}{-6}
=+15
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-11)+101}{2*-3}=\frac{112}{-6}
=-18+2/3
$