Suppose Rob's age is X and Jake's age Y
Since Rob is two times as old as Jake, X = 2Y
Since Three Years ago the sum of their ages was 21,
X + Y = 21 + 6 This is true because it was three years ago, so they both got older by 3 years
You plug in X to the second equation to fin 2Y + Y = 21 + 6 which is 3Y = 27, so Y = 7 Then, you plug this into the first equation and find that X = 14
So Rob is 14 and Jake is 7
Hope this helps :)