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