You have to use the Pythagorean Theorem. Follow what I do, and if you
have any questions be sure to tell me.
a^2+b^2=c^2
This is the formula. But don't get worried by the variables, you can
always switch a and b for l and m. Also c can be switched for n.
So not to get this confusing, I will substitute the letterings for you,
l^2+m^2=n^2
Lets now substitute the variables with the numbers.
12^2+5^2=c^2
169=c^2
Remember that n equals to the square root of 169.
n= √169
n=13
