Subtract the figures you've given from 20.9% (which is the percentage of oxygen in air, correct to 1 decimal place).
So, at rest:
20.9% - 19.0% = 1.9%
During exercise:
20.9% - 16.7% = 4.2%
Chris
PS: I've interpreted '19' as meaning '19.0' but you need to be aware that these are not the same for the purposes of data analysis. '19' means that the figure is closer to 19 than it is to 18 or 20. '19.0' means that the figure is closer to 19.0 than it is to 18.9 or 19.1. (i.e it defines a much smaller interval). Your data, as written, uses three different levels of accuracy (i.e. to the nearest integer, to the nearest tenth and to the nearest hundredth). If your question is related to a study at, say, basic GCSE level you might get away with such inconsistencies. However, for higher grade GCSE work, plus data analysis for AS levels and beyond, you need to ensure that you're using consistent figures.