Translating the given information directly into equations in two variables makes for more complicated equations to set up the problem and more complicated algebra to solve it. If you are going to use formal algebra, digest the problem before plunging into an algebraic solution to find an easier way to set the problem up.
If Mary was 9 years older than Elisha 6 years ago, then the difference between their ages is always 9 years. So
x = Elisha's age
x+9 = Mary's age
4 years from now the sum of their ages will be 49.
You could say that their ages 4 years from now will be x+4 and x+13 and solve the equation that says the sum of their ages will be 49 then. But I think it is easier to say that, since the sum of their ages in 4 years will be 49, the sum of their ages now is 41 (49 minus 4 for each of them).
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