8a 2a
--------- + --------
a^2 - b^2 a^2 + ab
You correctly factored the denominators, which is the first step in finding the common denominator:
8a 2a
----------- + -----------
(a + b)(a - b) a(a + b)
Now...what is the common denominator for the two fractions? It looks to me like the common denominator must contain these factors: a (a + b)(a - b)
Your next step is to rewrite each fraction as an equivalent fraction with a(a + b)(a - b) as its denominator. To accomplish this, you'll need to multiply numerator and denominator of the first fraction by "a", and numerator and denominator of the second fraction by (a - b):
8a(a) 2a(a - b)
----------- + -----------------
(a+b)(a-b)a (a+b)(a-b)a
NOW....the two fractions have the same denominator. ADD the numerators, and put the result over the common denominator.