If a Classical Cepheid star brightens by 0.8 mag without changing its effective temperature in a 45-day period,

(a) Calculate the fractional increase in its radius. (I have already answered this and my answer was 45%.)

(b) Calculate the apparent magnitude the star would have at the distance of the Virgo cluster. (This is the part I am having trouble, I am not sure which equation to use; distance modulus requires both apparent and absolute so if it is that equation I am unsure how to apply it with only the the brightness changing by a factor of 0.8 but not actually being given the magnitude.)

I saw the edit that says more information is needed but does not specify what...can an expert please assist on what I need to edit?