ABCD -square AB=BC=CD=DA
CEF -equilateral triangle, CE=EF=CF
CD=CE - given, CE=CF=CD
BC=CD, and CF=CD, so BCF is isosceles triangle
DCE is isosceles triangle, D=40, E=40,DCE angle =180-40-40=100 degrees
ECF=60 degrees because CEF -equilateral triangle
DCB=90 degrees because ABCD square
DCE+ECF+DCB +BCF=360
100+60+90+BCF=360
BCF=200-90=110
BCF is isosceles triangle,so 180=x+x+110, 2x=180-110, 2x=70, x=35