Explanation:
what's the problem ?
the perimeter is the sum of all 4 sides (imagine you are walking around the whole area - how long is the distance you are waking ?).
so, if I understand your picture correctly, we have the side lengths as
12.4
9
12.5
9
the sum of these 4 lengths and therefore the perimeter is :
12.4 + 9 + 12.5 + 9 = 42.9