What is the total mass of 14.2 grams

 When adding and subtracting different numbers of significant figures, the smallest decimal place of each of your terms is what your answer should look like: 

1) 34.61 - 17.3 = 17.31 , but because the term 17.3 only goes to one decimal place, this is the furthest the answer can go. 

So round up or down and you get 34.61 - 17.3 = 17.3 meters 

2) Same thing, but with three terms. 14.2 is the furthest decimal. 

14.2 + 8.73 + 0.912 = 23.842 

And again round up or down, so your answer is: 

14.2 + 8.73 + 0.912 = 23.8 grams