I don't think you're setting up your law of cosines correctly. Take a look at the wiki link, and tell me what the equation ought to be.
Yes, you can think. Your equation is this:
$\displaystyle 18^{2}= 10^{2}+11^{2}-2\times 10\times 11\times \cos(?)$
Simplifying one step leads to
$\displaystyle 324=100+121-220\cos(?)$
Your goal, as usual, is to get the $\displaystyle \cos(?)$ on one side of the equation and everything else on the other. What's your next step?