For concentric conducting shells, the potential at any point = sum of contributions from all shell charges. At the inner shell (radius a, charge Q1) with outer shell (radius b, charge Q2): $V_a$ = $\frac{kQ1}{a}$ + kQ2/b. $V_b$ = $\frac{kQ1}{b}$ + kQ2/b. If outer shell is grounded ($V_b$ = 0): Q2 = -Q1b/b... actually kQ1/b + kQ2/b = 0 gives Q2 = -Q1. If inner shell is grounded: kQ1/a + kQ2/b = 0, so Q1 = -aQ2/b. Grounding changes the charge distribution. The capacitance of the system: C = $\frac{Q}{V_a - V_b}$ = 4pi*$epsilon_0$*$\frac{ab}{b-a}$. These problems require careful bookkeeping of which shell has what charge and how grounding modifies charges.