40=X^2-16x

Solution for 40=X^2-16x equation:

40=X^2-16X

We move all terms to the left:

40-(X^2-16X)=0

We get rid of parentheses

-X^2+16X+40=0

We add all the numbers together, and all the variables

-1X^2+16X+40=0

a = -1; b = 16; c = +40;
Δ = b2-4ac
Δ = 162-4·(-1)·40
Δ = 416
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$X_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$X_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

The end solution:

$\sqrt{\Delta}=\sqrt{416}=\sqrt{16*26}=\sqrt{16}*\sqrt{26}=4\sqrt{26}$
$X_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(16)-4\sqrt{26}}{2*-1}=\frac{-16-4\sqrt{26}}{-2}
$
$X_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(16)+4\sqrt{26}}{2*-1}=\frac{-16+4\sqrt{26}}{-2}
$