Let the youngest be x years. The oldest will be 2x as they are twice the age of the youngest child. The middle child will be x+5 as they are 5 years older than the youngest. So the final equation looks like this:
x + 2x + (x+5) = 57
4x + 5 = 57
4x = 52
x = 13
So the youngest is 13, the oldest is 26 and the middle child is 18. To check, do 13 + 26 + 18 = 57