Question

(a) 9 cm + 2.8 cm = 11.8

Answer 1

 For addition and subtraction you round to decimal places. 
Round the number so that it has the same number of decimal places as the number in your data that has the least number of decimal places. 

9 cm + 2.8 cm = 11.8 cm 
the 9 has no decimal places so 
= 12 cm 
(this is rounded to 0 decimal places.) 
(It has 2 sig figs)