700=(3.9-0.1x)(120+20x)

Solution for 700=(3.9-0.1x)(120+20x) equation:

700=(3.9-0.1x)(120+20x)

We move all terms to the left:

700-((3.9-0.1x)(120+20x))=0

We add all the numbers together, and all the variables

-((-0.1x+3.9)(20x+120))+700=0

We multiply parentheses ..

-((+0x^2+0x+78x+468))+700=0


We calculate terms in parentheses: -((+0x^2+0x+78x+468)), so:

(+0x^2+0x+78x+468)

We get rid of parentheses

0x^2+0x+78x+468

We add all the numbers together, and all the variables

x^2+79x+468

Back to the equation:

-(x^2+79x+468)


We get rid of parentheses

-x^2-79x-468+700=0

We add all the numbers together, and all the variables

-1x^2-79x+232=0

a = -1; b = -79; c = +232;
Δ = b2-4ac
Δ = -792-4·(-1)·232
Δ = 7169
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-79)-\sqrt{7169}}{2*-1}=\frac{79-\sqrt{7169}}{-2}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-79)+\sqrt{7169}}{2*-1}=\frac{79+\sqrt{7169}}{-2}
$