You don't know how large N1 and N2 are. Maybe they are small numbers 2 or 3 or 10 or so, and the diameter of the wire is small compared to the other dimensions; therefore this is a good approximation. If the coils were solenoids you would need the length which you weren't given. Besides in an ideal solenoid there is no field outside the solenoid, so how could one coil produce a flux in the other coil if they are x apart? The geometric description just doesn't fit for solenoids. I've told you how to get the answer using the B-S law that you've studied. Do what I've told you, and you will get the answer. If you have other questions ask your instruct.