Question

9 cm + 2.8 cm = 11.8

Answer 1

It should be rounded to 2 significant figures since you cannot go to 1 significant figure if there is more than 1
So 2.8 has 2 significant figures so 11.8 should be rounded to 12

Answer 2

It depends on the precision you want.

Considering the data you've given, if you want the result in cm, the numbers should be rounded to 9 and 3, thus giving a total of 12, with two significant numbers.

But taking into account the precision you've reported, looks like the decimals do matter, and in that case, the numbers should be rounded to 9.0 and 2.8, giving a total of 11.8, thus, three significative figures.