a) To find the missing side length on triangle A, we can use the Pythagorean theorem.
a^2 + 12^2 = 13^2
a^2 + 144 = 169
a^2 = 25
a = 5 cm
5 and 12 = 5 and 12
b) The two triangles are congruent because they have two side lengths in common. If at least two side lengths are the same on two triangles, then they are congruent. In addition, these triangles have one angle in common as well.
Hope this helps!! :)