Question 913498
You found all you can in the description.  Nothing more is available in the description which you presented.


Lucy's age, x.
Little sister, x-3.
Older sister, 2x-5.


That is all.  Not enough information given for any equation.  Maybe, how much older is Lucy's older sister than Lucy?  But you still have a variable:  2x-5-x=x-5.