Can anyone help me find k or point me in the right direction?
3pisk+3pis^2=S
If the equation is:
$\displaystyle 3\pi\cdot s\cdot k+3\pi\cdot s^2=S$
$\displaystyle \Leftrightarrow 3\pi\cdot s\cdot k=S-3\pi\cdot s^2$
$\displaystyle \Leftrightarrow k=\frac{S-3\pi\cdot s^2}{3\pi\cdot s}$
You have forgotten a factor $\displaystyle s$ in the denominator.