The 24 students will form 12 pairs. The total number of magnets needed is 12*3 = 36. Mr. Sato gave out 9*3=27 magnets already and so he will need 36 - 27 = 9 more magnets.
I just took this class for Algebra 1... it would be -2
y=mx+b
this is the equation you will use for these problems. the "b" in the equation represents the y intercept. In this specific problem -2 is the y intercept