Please explain step by step

Spinning charged disk. A thin plastic disk of radius R has a charge Q uniformly spread over its surface. It is spinning with angular speed a; about its central axis. (a) In terms of M) and the given symbols, what is the magnitude of the magnetic eld at the center of the disk? (Hint: The rotating disk is equivalent to an array of current loops around the center.) (b) What is the magnetic dipole moment of the disk