In short the answer is that the width is 19.
Explanation:
We can solve this with an equation. If W is the width then the length is W + 6. We can set up an equation and solve it as follows:
2W + 2(W + 6) = 88
2W + 2W + 12 = 88
4W + 12 = 88
4W = 76
W = 19