smc

We start off by factoring the second fraction. $$-\frac{ab-b^2}{ab-a^2}=-\frac{b(a-b)}{-a(a-b)}=\frac{b}{a}.$$ Now create a common denominator and simplify. $$\frac{a^2-b^2}{ab}+\frac{b}{a}=\frac{a^2-b^2}{ab}+\frac{b^2}{ab}=\frac{a^2}{ab}=\boxed{\frac{a}{b}}$$ obs: Assume that $a\ne 0,b\ne 0,a\ne b$.