order of operations (PEMDAS):
21 - 18 / 3 * 2
let's add some parenthesese to make it easier to see:
21 - (18 / 3 * 2)
multiplication and division are associative
also: 18 / 3 = 18 * (1/3)
21 - (18 * (1/3) * 2)
21 - (6*2) or 21 - (36 * (1/3)) or 21 - (18 * (2/3))
21 - 12
9