An arithmetic series has a first term of $\displaystyle -4$ and a common difference of $\displaystyle 1$. A geometric series has a first term of $\displaystyle 8$ and a common ratio of $\displaystyle 0.5$. After how many terms does the sum of the arithmetic series exceed the sum of the geometric series?

I end up getting:

$\displaystyle \frac{n}{2}(n-9)>4(1-0.5^n)$

and I'm not sure how to solve it!

The answer says n=12.