It is given that AX = AY
Considering △ AXC and △ AYB
∠ A is common i.e. ∠ A = ∠ A
We know that the sides of equilateral triangle are equal so we get
AC = AB
By SAS congruence criterion
△ AXC ≅ △ AYB
XC = YB (c. p. c. t)
Therefore, it is proved that CX = BY.