Please help me I don't understand this?

The "2y" and "y" represent the stoichiometric ratio between I2 and I. For every one mole of I2 in solution, there are two moles of I (from the balanced equation). So if the change in I2 is (-y) then the change in I must be (-2y). You were already given the "E" for each in the "ICE" chart. Solve for the change, or y.