Question 31018
I am going to be using s to represent son and d to represent daughter.
s=son
d=daughter

From the first part,"(the son) (is) (three years) (older) than (the daughter)"
                         s       =      3            +           d 
Then, "(In 17 years) (the son)(will be)(twice)as old as(the daughter is now.)" 
             17    +    s         =       2                     d
Keep in mind that the daughter has not "aged". We know this by "the daughter is now"

You get the two equations
s=3+d
17+s=2d

and you substitute the equations
PART ONE
17+(3+d)=2d   
17+3+d=2d     add
20+d=2d       divide
20=d 
 PART TWO
s=3+(20)
s=23          add

So the son is 23 now, in 17 years he will be 40, which is twice the age of the daughter, 20.