Chris is 2 years old and jenny is 4 years old. in how many years will jenny be 7 years younger than twice chris' age?

Let x be the unknown number of years.
Let C be Chris' age and J be Jenny's age.
Then J+x = 2(C+x)-7
    ... describes this situation.
          4+x = 2(2+x) - 7
           4+x = 4 + 2x - 7
               x  =  2x -7         becomes 7=x

In 7 years, Jenny will be 7 years younger than twice Chris' age.

Check:  Is this true?     4+7 = 2(2+7)-7
                                       11  =  18-7, or 11=11               TRUE