If it is 76 feet LOWER than 73 above sea level, then you must do
73-76= -3
So Mudville is 3 feet BELOW sea level.
T,A,Y are 40 and 2T,X,B,Z are 80
Sina - (cosa)(tanb)/cosa + (sina)(tanb)
sina ≡ (tana)(cosa)
(tana)(cosa) - (cosa)(tanb)/cosa + (tana)(cosa)(tanb)
= cosa(tana - tanb)/cosa(1 + tanatanb)
(cosas cancel out)
= (tana - tanb)/(1 + tanatanb) ≡ tan(a-b)