x2-15x=42

Solution for x2-15x=42 equation:

x2-15x=42

We move all terms to the left:

x2-15x-(42)=0

We add all the numbers together, and all the variables

x^2-15x-42=0

a = 1; b = -15; c = -42;
Δ = b2-4ac
Δ = -152-4·1·(-42)
Δ = 393
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{-(-15)-\sqrt{393}}{2*1}=\frac{15-\sqrt{393}}{2}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-15)+\sqrt{393}}{2*1}=\frac{15+\sqrt{393}}{2}
$