Comparing lists

I have two sets of data. Each set has a column of part numbers and their respective quantities in the next column (Col A: Pt No. Col B: Qty).
Many of the Part numbers are common to each data set and what I want to do is compare the part numbers and look at the difference in the quantities. What is the simplist way of doing this please?

Thank you

I think using vlookup is rather simple and straight forward for your case