Set up your diagram with 3 circles, S, G, A, for each subject. The middle should have a small space that includes part of all 3 circles. Start from this spot and move outward.
This very middle spot represents the students who are in all 3 subjects. As given in the problem, there are 15, so write that in the center portion. Now lets try to fill out the rest of the Geography circle.
The next portion of G we can find is the # of students who are in both Geography and Science but NOT art. You are given that 35 students are in G and S, however 15 of these students are ALSO in art. To get those only in G and S, subtract the 15 from the entire 35 taking both subjects. This gives us 20 students taking G&S but NOT A.
Next is the students taking G and A, but NOT S. It is given that 33 students are taking G and A. Again, 15 of these are ALSO in Science, therefore subtract the 15 from the entire 33 to get 18 students in G&A only.
Now we have everything but the students taking STRICTLY geography. You are given that there are 70 students total in G, and have found 20 are in G&S, 18 in G&A, and 15 in G&S&A. Subtract these students from the entire 70 to get 17 that are only taking Geography.
Now do the same procedure with the circles for Art and for Science. At the end, you can sum all of the sections you have filled out to find the total # of students enrolled in the subjects and subtract that # from 200 to find those not in any of them.