The two base angles in each triangle are equal. (The triangle is isoceles and the property used is angles opposite equal sides are equal).
Now the tricky part. The peak angles are also equal. That's because both peak angles are made equal by Peak angle + 2*base angle = 180
Peak angle = 180 - 2*base angle
Therefore the triangles are congruent by SAS.
I suppose you could get them equal by taking one of the base angles (all 4 are equal to each other) and the peak angles and claim equality by ASA, but it seems a little bit tortuous to me.
