From what is given within the question, it appears that this is a question which requires one to exploit Beer's Law shown below:
A = k c
A is measured absorbable of a sample with concentration c where k is a constant.
Given a standard solution with absorbable As having a known concentration Cs AND a diluted cholesterol solution with absorbance Aunk with an unknown concentration Cunk, the following is true:
Cs / As = Cunk / Aunk = k
This assumes that the measurements were taken at the same wavelengths using a sample cuvette with a path length of 1.0 cm.
Using the give data, we find that ...
3.9 mM / 0.77 = Cunk / 0.28
Cunk = ( 3.9 mM / 0.77 ) • 0.28 = 1.4 mM
Thus, the concentration of cholesterol within the 4-fold dilution that was measured is 1.4 mM. One would then have to scale up this result 4-food to acquire the predicted concentration of cholesterol in the original undiluted sample.