1980=(330+15x)(6-0.25x)

Solution for 1980=(330+15x)(6-0.25x) equation:

1980=(330+15x)(6-0.25x)

We move all terms to the left:

1980-((330+15x)(6-0.25x))=0

We add all the numbers together, and all the variables

-((15x+330)(-0.25x+6))+1980=0

We multiply parentheses ..

-((+0x^2+90x+0x+1980))+1980=0


We calculate terms in parentheses: -((+0x^2+90x+0x+1980)), so:

(+0x^2+90x+0x+1980)

We get rid of parentheses

0x^2+90x+0x+1980

We add all the numbers together, and all the variables

x^2+91x+1980

Back to the equation:

-(x^2+91x+1980)


We get rid of parentheses

-x^2-91x-1980+1980=0

We add all the numbers together, and all the variables

-1x^2-91x=0

a = -1; b = -91; c = 0;
Δ = b2-4ac
Δ = -912-4·(-1)·0
Δ = 8281
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{8281}=91$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-91)-91}{2*-1}=\frac{0}{-2}
=0
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-91)+91}{2*-1}=\frac{182}{-2}
=-91
$