We add all the slices together by working out how many slices each person ate: Eddie and Melanie ate 1 slice each; Mom ate 2 and Dad ate 3; Albert ate 2. That's 1+1+2+3+2=9 slices, leaving 3 slices out of the 12. As an example of converting the fractions into slices: Dad ate 1/4 and 1/4 of 12 is 12/4=3.
The problem can also be solved by adding all the fractions together: 1/12+1/12+1/6+1/4+1/6=(1+1+2+3+2)/12=9/12. 9/12 of a pizza cut into twelve slices is 9/12*12=9.