Well, I'm new here, but I'd be happy to help you out.
You
did say the external pressure was constant and not the internal pressure, but the fact that the external pressure is constant is important!
Personally, I think it's important to look at these types of problems conceptually. If you start out at 15 atm and 10 L, and you know that the gas is expanding, when does it stop expanding? It doesn't expand forever, right?
So if, in fact, you know that the external pressure is a constant 2 atm, then
that is the final pressure, both internal and external; reason being is that the overall system must come to equilbrium. The internal pressure of the gas shouldn't exceed the external pressure (unless it is heated) and it also shouldn't go below it -- if it does go lower than 2 atm, it contracts!
Now that you know that P2 = 2 atm, you can use Boyle's law to solve for V2, and from there calculate the work done.