IF you can trust the headers
then the resolution is 2835 pixels per meter, which happens to correspond to 72 pixels per inch.
However, when you see 72 pixels per inch or 100 pixels per inch, you should pretty much assume that nominal values have been written into the headers, that might have no relationship to reality.
There is probably nothing in those headers that you should trust other than the height and width.
You either need external information (e.g., knowledge that it was taken with a microscope with particular settings), or else you need to know the true size of some object in the field of view in order to calculate scales.