Magnetic field using Biot-Savart's Law

[K.QMUL]

Homework Statement

An infinitely long hollow cylinder of radius a carries a constant current I. Use Biot-Savart's law
and show that the magnetic field at the centre of the cylinder is zero.

Homework Equations

Biot-Savart's law

The Attempt at a Solution

I cannot seem to come up with anything meaningful. I have tried searching this question but it seems the best way is normally to use Ampere's law and not Biot-Savarts law. Could someone explain the steps or show the steps how we could use Biot-Savart's law in this case.

 

[tman12321]

This is really just a symmetry argument. Think about sitting between two wires that have current flowing in the same direction. You know the magnetic field from one wire (using the Biot-Savart law), and you know it is going to cancel that of the other wire in the middle. Now just line the surface of a cylinder with an infinite number of these wires and you have your problem, but you get the same result.