18=12n-n^2

Solution for 18=12n-n^2 equation:

18=12n-n^2

We move all terms to the left:

18-(12n-n^2)=0

We get rid of parentheses

n^2-12n+18=0

a = 1; b = -12; c = +18;
Δ = b2-4ac
Δ = -122-4·1·18
Δ = 72
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$n_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$n_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

The end solution:

$\sqrt{\Delta}=\sqrt{72}=\sqrt{36*2}=\sqrt{36}*\sqrt{2}=6\sqrt{2}$
$n_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-12)-6\sqrt{2}}{2*1}=\frac{12-6\sqrt{2}}{2}
$
$n_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-12)+6\sqrt{2}}{2*1}=\frac{12+6\sqrt{2}}{2}
$