x(x-3)-7(x+3)=0

Solution for x(x-3)-7(x+3)=0 equation:

x(x-3)-7(x+3)=0

We multiply parentheses

x^2-3x-7x-21=0

We add all the numbers together, and all the variables

x^2-10x-21=0

a = 1; b = -10; c = -21;
Δ = b2-4ac
Δ = -102-4·1·(-21)
Δ = 184
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:

$\sqrt{\Delta}=\sqrt{184}=\sqrt{4*46}=\sqrt{4}*\sqrt{46}=2\sqrt{46}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-10)-2\sqrt{46}}{2*1}=\frac{10-2\sqrt{46}}{2}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-10)+2\sqrt{46}}{2*1}=\frac{10+2\sqrt{46}}{2}
$