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To get a six-pack, you must exercise your upper abs, lower abs, and obliques (side abdominals). While you cannot target each area exclusively, each abdominal exercise emphasizes a particular area. The following exercises will help get you started. Plank exercises are important for any abdominal muscle workout because they work all the ab muscles together, plus many other muscle groups. To do this exercise, get in push-up position, using your elbows for support rather than your hands. Keep your body in a straight line, making sure that your hips do not sag. Contract and pull in all the muscles around your middle, and do not let your belly drop. Hold for as long as you can.  Keep your head relaxed and looking at the floor.  Start off holding this position for 10 seconds at a time, and work your way up to longer intervals.  To make sure that your body is straight, do this exercise in front of a mirror.
Target all three abdominal areas. Do planks.