First, you need to calculate the standard cell potential using standard reduction potential from a textbook or online. Since Mg becomes Mg+2, magnesium is being oxidized because it is losing electrons, you need to flip its potential
Fe+2 + 2e- --> Fe potential= -0.44
Mg+2 + 2e- --> Mg potential= -2.37
Cell potential= (-0.44) + (+2.37)= 1.93 V
Now, you need to use Nernst formula to get the answer. I have attached a PDF with the work.
