it is -15.4 they're both negative numbers (-7.7 and -7.7, when you look at them separately) and you just add them up, but keeping the - sign in front of the sum, because: -7.7 - 7.7 = - (+7.7 + 7.7)...
-7.7 + 7.7 = 0
154
-15.4
0 the reason of this is if you take away 7 from 7, you dont have anything left. =] hope i helped!