It looks like you are asking for a simplification involving the variables with units of meters (m) and possibly a different variable (dm typically represents decimeters). However, the expression provided is unclear.

If you meant \(8m + 5dm = ?\) and \(8m - 5dm = ?\), we need to convert decimeters to meters to do the operations correctly.

1 decimeter (dm) = 0.1 meters (m)

So, we convert \(5dm\) to meters:

\[
5dm = 5 \times 0.1m = 0.5m
\]

Now let's simplify the expressions.

1. For \(8m + 5dm\):
\[
8m + 5dm = 8m + 0.5m = 8.5m
\]

2. For \(8m - 5dm\):
\[
8m - 5dm = 8m - 0.5m = 7.5m
\]

Thus, if that was your intent:
- \(8m + 5dm = 8.5m\)
- \(8m - 5dm = 7.5m\)

If you meant something else, please clarify your question!