Tom's and tim's ages add up to 21 years. in 3 years, tom will be twice time's age. how old are they now?

X=Tom´s age
y=Tim´s age.
we suggest this system of equations: 

x+y=21
(x+3)=2(y+3)

We solve this system of equations by susbstitution method.
x=21-y
[(21-y)+3]=2(y+3)
24-y=2y+6
-y-2y=6-24
-3y=-18
y=-18/-3=6

x=21-y
x=21-6=15.

Tom is 15 years old, and Tim is 6 years old.

What's the denominator