4м-27дм+(8м 6 дм-5м 9см)

To solve this expression, we need to simplify and combine like terms.

First, let's add the lengths: (8m 6dm) + (-5m 9cm)

To add the lengths, we have to convert the measurements to the same unit. Since we have meters (m) and decimeters (dm), we can convert decimeters to meters.

1 decimeter (dm) is equal to 0.1 meter (m).

So, (8m 6dm) + (-5m 9cm) = (8m + 0.6m) + (-5m 0.9m)

Simplifying further, we have: 8.6m + (-5.9m)

Now, let's combine the terms with the same units: 8.6m + (-5.9m)

To add these, we add the coefficients (numbers in front): 8.6 + (-5.9) = 2.7

So, the final answer is 2.7m.