Answer is 13.8
Step by step
Your constant of proportionality is the slope or rate of the graph.
The constant of proportionality k is given by k=y/x.
From your graph you have points given ( 2, 27.60) and ( 6, 82.80)
k = 27.60/2 = 13.8
k = 82.80/6 = 13.8
Rate is the same and the line goes through (0, 0) so it is proportional