If you would like to simplify Q + S - T, you can
do this using the following steps:
Q = 7m + 3n,
R = 11 - 2m,
S = n + 5,
T = -m - 3n + 8
Q + S - T = 7m + 3n + n + 5 - (-m - 3n + 8) = 7m + 3n + n + 5 + m + 3n - 8 = 8m + 7n - 3
The correct result would be 8m + 7n - 3.
