Watch this five-minute video. Pay attention to the examples used to explain adding and subtracting when there are different denominators. Here, one would first obtain a common denominator for the expressions involved and then proceed as in subunit 5.4.2.1 above.
You may watch these MathIsPower4U videos if you feel you need additional help on this topic.
"Ex 1: Add and Subtract Rational Expressions - Like Denominators"
"Ex 2: Add and Subtract Rational Expressions - Like Denominators"
"Ex 3: Add and Subtract Rational Expressions - Like Denominators"