Well... with the information you are given you just have to assume they are congruent triangles. The first picture looks like a right triangle, but the angle is 100 not 90... so it wouldn't actually look like that.
So for the missing angle all you do is 180= 100+ 55 + x
x=25
Then just match up the angles with the other triangle, they would be the same. K with F, J with G, and so on
