Answer: 5m
Explanation:
The bottom of the diagram shows a length of 15m so the top of the diagram needs to equal 15m.
Hence you are given lengths of 5m, ?, 5m so turn this into an algebra question and you will have (5m + x + 5m = 15m) and solve for x.
10m + x = 15m
x= 15m - 10m
x= 5m