Rotational Newton's second law problem?

A pulley has an inner radius of 0.35m and an outer radius of 0.65m. It has a mass of 1.8kg. A string wrapped around the inner part of the pulley is fastened to the ceiling. a second string wrapped around the outer part of the pulley is fastened to a block. Determine the mass of the block if the pulley remains at rest. You should get and answer of 2.1kg but I want to know how you got that please.