When Tom found found the difference - 11 - (-4) he got an answer of - 15. What might Tom have done wrong?

Tom combined like terms as if it were an equation instead of solving the problem. - 11 - (-4) would turn into a - 11+4 and to solve that do 11-4=7 but the 7 is negative because the bigger number in that problem (11) was negative. - 11 - (-4) = -7

Tom apparently added - 4 to - 11 instead of subtracting it.