Each letter stands for a number 0 -9 but not all digits will be used.
983 + 86 + 8 = 1077
writing it vertically helps a lot
The B has to be a 1 as the total would be over 1000 and the M has to be a 9 as we are only adding a two digit and 1 digit number and getting over 1000
the last column must be adding up to 17 rather than 7 (otherwise S + A would be 4 and would be 1 and 3 but 3 is already used)
so S + A = 14 and since there will be a carry the next column over A + A + 1 will equal 17
so A = 8 and therefore A = 6