Find two unknowns by the elimination method. First MULTIPLY one (or both) of the equations before ADDING or SUBTRACTING them together. It's much easier than it sounds!
If any of the examples are too easy for you skip to the next one...
Example 2 begins at 4:20
Example 3 begins at 8:21
Example 4 begins at 13:03
Example 5 begins at 16:48