Check the picture below.
we know that ∡A = ∡B, because AC = BC and twin sides in an isosceles make twin angles, so we can say that ∡CAB = ∡ CBA, and we also know that the inner triangle is also an isosceles, so we know that ∡DAB = ∡EBA.
Now, as you can see in the picture below, if we subtract an equal angle quantity from both ∡CAB and ∡CBA, whatever is leftover, since both were equals to begin with, must be equal leftovers.