set the sisters' ages to x and y
x + y = 35
let's say x = younger sister's age
and y = older sister's age
so
x = [(1/3)*y] + 3
plug in the value above for x in the equation x + y = 35
solve for y, then for x
remember which variable you set for the younger sister, and make sure that you use that variable as your answer
Hope this helped!