Answer:
All angles must add up to 180, so if you add 69+5x+2+9y-3, it is = to 180.
First we can subtract 68 cancelling out 69, 2 and -3 so it is
5x+9y = 112. I assume this is an iscoceles triangle so we already knew 9y - 3 = 69, y=8, x=8
43n^2-54. then do the exponent part your self! :)
I believe that the missing letter would be E and S which would be in between S and T
