C(x)=x2-90x+1400

Solution for C(x)=x2-90x+1400 equation:

(C)=C2-90C+1400

We move all terms to the left:

(C)-(C2-90C+1400)=0

We add all the numbers together, and all the variables

-(+C^2-90C+1400)+C=0

We get rid of parentheses

-C^2+90C+C-1400=0

We add all the numbers together, and all the variables

-1C^2+91C-1400=0

a = -1; b = 91; c = -1400;
Δ = b2-4ac
Δ = 912-4·(-1)·(-1400)
Δ = 2681
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$C_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$C_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$C_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(91)-\sqrt{2681}}{2*-1}=\frac{-91-\sqrt{2681}}{-2}
$
$C_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(91)+\sqrt{2681}}{2*-1}=\frac{-91+\sqrt{2681}}{-2}
$