You may want to review simplifying expressions with fractional exponents first.
If there are additions or subtractions involved, then you can't necessarily combine the two terms. All you can do is to take out a common factor, or look for a common denominator. For example, suppose you're given 1 ⁄ 5 − (−2)a−1 ⁄ 3b2c. You can take out the common denominator 5a1 ⁄ 3. So the answer is (a1 ⁄ 3 + 10b2c) / (5a1 ⁄ 3).
Remember, the answer should have no negative exponents.