equilateral means those 3 angles are 60 degrees
ok, so we got that the left polygon is 10 sides
so the internal angles are 144 degrees in those angles
360=around
CAD+CAB+DAB=360
CAD+60+144=360
CAD=156 degrees
the measure of an interior angle of an ngon is (n-2)*180/n
156=(n-2)(180)/n
solve for n
times both sides by n
156n=180n-360
minus 180n both sides
-24n=-360
divide both sides by -24
n=15
15 sided
n=15