from the two different formulas for energy one can find that the g-factor equals \frac{\mu}{I \mu_N}
a) for the ground state g = -1.1897 (I = 1/2 and \mu = -0.59485 \mu_N)
for the excited state g = -0.3064 (I = 5/2 and \mu = -0.766 \mu_N)
b) we find g = 2 (I = 1/2 and \mu = 1 \mu_B) (here we have to divide by \mu_B instead of \mu_N)
c) the other way around \mu = I g \mu_N = -1.913 \mu_N (I = 1/2)