Answers:
(a) 7c
(b) 21
============================
Step-by-step explanation:
Start with 9c and subtract off 2c to get 9c-2c = 7c.
We can think of it like 9 candies - 2 candies = 7 candies. Replace each "candies" with "c" so things are shortened.
Afterward, plug in c = 3 to find that 7c = 7*3 = 21