let x=Don's present age
2x=Chris's present age
next year:
x+1=Don's age
2x+1=Chris's age
(x+1)+((2x+1)=4(x-1) (next year the sum of their ages will be four times as much as Don's age last year.)
x+1+2x+1=4x-4
x=6
2x=12
Don's present age=6
Chris's present age=12