The key is to write down the two equations that are implied here:
First, Sara (S) is 25 years older than Teresa (T): S = 25 + T
Second, in 4 years S will be twice as old as T: S + 4 = 2(T+4).
The last equation can be solved for S, which looks, after distributing, like:
S = 2T + 4
You can use substitution of this equation into the other to solve for T and then use that value to solve for the value of S.
