Equilibria: describe question

The reaction $\ce{SO2(g) + 1/2O2(g) <=> SO3(g)}$ is exothermic in the forward direction. A mixture of sulfur dioxide and oxygen is sealed in a rigid container and allowed to reach dynamic equilibrium at a fixed temperature.

A small amount of a solid catalyst is then added to the mixture, and the temperature is kept constant.

Describe, in terms of the rates of the forward and reverse reactions, what happens when the catalyst is added and how the position of equilibrium is affected. Explain why the catalyst has this effect.