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
