Question 52258
You've got a good start, but don't introduce a 2nd variable here.  You've got x as the Michigan plant -- good; and 4x is the age of the Ohio plant -- very good.  The difference in their ages is 9; that means that {{{4x-x=9}}}.  Solving for x, we get {{{3x=9}}}, which simplifies to {{{x=3}}}.  That means the Michigan plant is 3 years old and the Ohio plant is 4 times that, or 12 years old.  Note the difference in ages is 9.