Answer:

s = 9

Step-by-step explanation:

Joseph's age now ( j) = 18

Sam's age 3 years later (s + 3) = j - 6

s + 3 = j - 6

s + 3 = 18 - 6 ; substitute j = 18

s + 3 = 12

s = 12 - 3

s = 9