Ratio Forumla

I have a difficult math equation and was able to solve it, BUT I DON'T KNOW HOW TO PUT IT IN EQUATION FORM. i have listed the problem below. does anyone know an equation to solve it, or is it just guess and check?

Q:
john is twice as old as his son. in 42 years, the ratio of their ages will be 4:3. what is john's son's age now?:confused:

If you give us the answer I'll be happy to give you the equations and solve them. Interestingly, guess and check amounts to walking along a line in a plane and doing an intersection test at every point. I'll be happy to explain what I mean as soon as I know this isn't a homework problem.