M = Mell
J = Jerry
M = 2J [Mell is twice as old as Jerry]
M - 16 = J + 16 [When Mell's age is subtracted by 16, and 16 is added to Jerry's age, their ages will be equal]
Since you know M = 2J, you can plug in/substitute 2J for "M" in the second equation:
M - 16 = J + 16
2J - 16 = J + 16 Add 16 on both sides
2J = J + 32 Subtract J on both sides
J = 32
Now that you know Jerry's age, you can find Mell's age:
M = 2J
M = 2(32)
M = 64
Mell is 64, Jerry is 32