well, let's take a peek, hmmm the length is hmmm, how much is five times "x"? well 5 * x or just 5x, how about four more than that? well, just 5x + 4.
we know the width is hmmm how much is three times "x"? 3*x or just 3x, how about 2 less than that? 3x - 2.
Check the picture below.