There are 360o around each point.
Subtract the 72o to get 288o.
These degrees will be equally divided between the two polygons: 144o.
Using the formula for the number of degrees in each interior angle of a regular polygon:
[ (n - 2)·180o ] / n = 144o
Solving: (n - 2)·180o = 144o·n
180o·n - 360o = 144o·n
36o·n = 360o
n = 10