you can solve these by substitution. so your equations are:
15.49x = y
13.99x + 6 = y
because they both equal y, you can set them equal to one another:
15.49x = 13.99x + 6
subtract both sides by 13.99
1.5x = 6
divide both sides by 1.5
x = 4
so you can buy 4 cds for the same amount from each store. you can check it by plugging the x in:
15.49x = y
13.99x + 6 = y
15.49(4) = y
61.96 = y
13.99(3) + 6 = y
55.96 + 6 = y
61.96 = y
both would cost you $61.96