What is the correct answer for the subtraction 28.645 g - 22.105 g? (considering significant figures) ?

The answer is 6.54 when you calculate it. However, since out of the two numbers you used in the calculation, the number with the least number of decimal places has to be used for the answer, as well, which in this case is 3 decimal places. So, since 6.54 only has two decimal places and you need to make it three, just add a zero the end, thus making it 6.540. Final answer to the correct significant figures: 6.540