Check the image attached for reference.
Now we put "Six years from now,he will be twice as old as Ronnie." as an equation.
It will be Rafael=Ronnie*2, which is also:
x+16 = 2(x+6)
Then, we get:
x+16 = 2x+12
Doing the necessary transposing, we get x as 4.
Hope that helps.