4м-27дм+(8м 6 дм-5м 9см)

To solve this expression, we need to add and subtract the quantities of meters, decimeters, and centimeters separately.

First, let's focus on the meters: 4m + 8m = 12m

Next, let's concentrate on the decimeters: -27dm + 6dm = -21dm

Now, let's deal with the centimeters: -5cm + 9cm = 4cm

Finally, let's put it all together: 12m - 21dm + 4cm

However, we need to make sure all the units are consistent. Since 1m = 10dm and 1dm = 10cm, we need to convert the decimeters and centimeters to meters:

-21dm = -2.1m 4cm = 0.04m

Now, we can rewrite the expression in meters:

12m - 2.1m + 0.04m

Simplifying further, we get:

9.94m

Therefore, the final answer is 9.94 meters.