Start with the the fact the magnetic field B inside an infinitely long solenoid of arbitrary radius is
B = 4 π x 10-7 nI Tesla
Where n = turns of wire per meter, and I is the current in amps.
I know this rule too
i have seen a question talks about magnetic field , the question is solenoid inside a circuit inside two wires, it gives amps , turns , all the information that needed , but the length of solenoid is missed and needed to find it , and ifcousre find the Tesla in the center