Lindsey, this is the solution to this interesting problem:
R1 = 7 (People who don't eat hamburguers, pizza or tacos)
R2 = 12 (People who only eat hamburguers)
R3 = 5 (People who eat hamburguers and pizza)
R4 = 14 (People who only eat pizza)
R5 = 7 (People who eat hamburguers and tacos)
R6 = 2 (People who eat hamburguers, pizza and tacos)
R7 = 8 (People who eat pizza and tacos)
R8 = 15 (People who only eat tacos)
Therefore,
a. The answer is 14. (R4)
b. The answer is 21 (R1 + R4)