What is the concentration of magnesium ions?

A galvanic cell is constructed with a copper electrode dipping into a 0.015 M solution of Cu2+ ions, and a magnesium electrode immersed in a solution with an unknown concentration of magnesium ions. The cell potential is measured as 2.79 V at 25 °C. What is the concentration of magnesium ions? (numerical solution: 2.9×10-5 M)