Answer
You would write the number describing the total volume as 2 significant figures (e.g. 3.2). The precision of the number is limited by the lowest number of significant figures in the numbers used in the original calculation.
Work Step by Step
To find the total volume using correct significant figures:
Graduated cylinder 1 shows $\approx 2.9$ mL
Graduated cylinder 2 shows $\approx 0.28$ mL
The lowest number of significant figures between both numbers is 2. Therefore the sum of the two numbers should be written as 2 significant figures when the volumes are added to the beaker.
$2.9$ mL + $0.28$ mL $\approx 3.2$ mL
