there is a sign missing, but I think it's a "+" like this:
8-(2+5)
H, ,that is Harry's answer would be:
first the bracket:
8-(7)
and then the whole expression:
8-(7)=1, so H=1
Terry's answer would be:
8-2+5, which we do from left to right:
6+5=11, so T=11
then H-T is 1-11, which is -10.