The answer is 4.
x - the number of DVDs
You can buy DVDs at a local store for $15.49 each:
15.49x
You can buy them at an online store for $13.99 each plus $6 for shipping:
13.99x + 6
How many DVDs can you buy for the same amount at the two stores:
15.49x = 13.99x + 6
15.49x - 13.99x = 6
1.5x = 6
x = 6 : 1.5
x = 4