First, we need to find a common denominator for the two fractions. The smallest number that both 13 and 26 can divide into is 26.

-2/13 can be rewritten as -4/26 (multiply the numerator and denominator by 2).

-11/26 stays the same.

Now we can add the two fractions:

-4/26 + (-11/26) = -15/26

Therefore, -2/13 + -11/26 = -15/26.