The general relationship to solve problems like this is to recognize that:
L * M = L* M
this is true because if you multiply M * L you get moles. So what this is saying is that the number of moles is equal before and after.
So for your problem:
you have 14 M HNO3 and need to know the volume.
You want to end up with 2 L of 0.15 M HNO3
Substitute your knowns into the above equation and solve.