.
W + L + W + L = 58, or
2W + 2L = 58, or
2*(W + L) = 58, or
W + L = 58/2 = 29, or
W + (W+13) = 29,
2W = 29-13 = 16
W = 16/2 = 8.
ANSWER. W, or the width, is 8 inches.
In more short form
W + L = 58/2 = 29
W + (W + 13) = 29
2W = 29-13 = 16
W = 16/2 = 8.
Solved.