Chemistry 2 dilution problem

[shemer77]

1. The problem statement, all variables and given/known data
How much 2M HCl do you have to add to 5 liters of 6 M HCl, to obtain a 3.5 M HCl solution?

3. The attempt at a solution
I initially thought of something like m1v1=m2v2, but that doesn't seem to really work in this case :/?

[symbolipoint]

How many moles of HCl do you need in the target solution when finished, and how much volume must it be?

Let V = volume of 2M HCl to add. Final solution volume to make is V+5 liters.
Moles of HCl in final prepared solution are 2*V + 6*5 moles.

Analyze the situation and construct the needed equation and solve.