Round to the place value of the less precise measurement.

2.13 m + 8.6 m

The answer would be 10.19m. If you want to round it of to a less precise measurement it would be 10 m. That is rounding of the decimal value to the ones place. the rule of rounding of is form 6 t0 9 it would add 1 to the nearest place value next to it and 0 - 5 will not.