The width is 9.5.
First you know what the length is. You must find the measurement of both combined.
12.4 x 2 = 24.8 (Length)
Subtract this number from the perimeter. 43.8 - 24.8 = 19.
19 is your width multiplied by two. The total measurement of both, to find just the width divide by 2.
19 / 2 = 9.5 (WIDTH)