I strongly recommend that you enclose all those numerical fractions inside parentheses:
(1/2)m -(3/4)n =16 Otherwise there's doubt whether you meant m/2 or 1/(2m)
Mult all 3 terms by 4 to elim. fractions: 2m -3n = 64
Now let n = 8: 2m - 3(8) = 64, or 2m = 88, or m = 44 (answer)