For the first isotope, we know µ_1 and I_1. We can measure the “a” value. This gives us info about the B_J / J value for this isotope. This is a property of the electron cloud, so it will be the same for the 2nd isotope.
The spin I_2 is again given. The B_J / J value is known from the experiment on the 1st isotope. We can again measure the value “a” for our isotope.
µ = (IJa)/(B_J) will tell you the result for µ_2