$\text{Magnetic Field due to larger loop at its centre } B = \frac{\mu_0 I}{2R_1}$

$\text{Flux Passing through smaller loop is } \phi = B.A cos0^o = BA = \frac{\mu_0 I\pi R^2_2}{2R_1}$

$ \phi = LI = \frac{\mu_0 I\pi R^2_2}{2R_1}$

$ L = \frac{\mu_0 \pi R^2_2}{2R_1}$