Khan Academy
Khan Academy: Arithmetic and Pre Algebra: Adding Fractions With Different Signs
This video shows how to add three fractions, one of which has a negative sign. Practice problems are available. [6:22]
Khan Academy
Khan Academy: Arithmetic and Pre Algebra: Adding and Subtracting Fractions
This video shows how to add and subtract fractions with like and unlike denominators. Practice problems are available. [9:49]