In the statement the phrase the sum of can tell you that it is going to be an addition problem. So 8x ( eight times a number) would be added to the 15, 8x+15. Since its an addition problem the part where it says ' ... is seven', you know that it will all equal 7. So the answer would be 8x+15=7.
The triangles would be congruent, because the lines GH and FI are congruent and the angle H and I are the same, and since the line HI is shared by the two, it would then be SAS.
