These used to confuse me, too!
j = 3k
k = j - 16
Now, we have what j equals and what k equals. We can then substitute either into the other equation.
j = 3(j - 16)
j = 3j - 48
-2j = -48
j = 24
k = 24 - 16
k = 8
Orrr...
k = 3k - 16
-2k = -16
k = 8
j = 3(8)
j = 24