So basically the a's and s' mean angle and side. this means that if there is a line on both of the triangles on one side this means that those sides of the triangle are congruent. this is the same case with the angles. if you were to have one congruent side inbetween two congruent angles, you would use ASA, which stands for angle side angle, to prove that the triangles are congruent. some more examples are:
side, side, angle =SSA.
angle, angle side = AAS. (the problem in question)
