The formal Algebra way is to write this equation (9+x)*(12+x) = 154 and find x = "in how many years . . . ", solving this quadratic equation. As an alternative, you may notice that 154 is a multiple of 11, so 11, probably, will be the age of the younger sister. If 11, then it will happen in 2 years, when the older sister will be 12+2 = 14. So, you check 11*14 = 140+14 = 154, and you see that it is correct. Thus "2 years" is your ANSWER.