Why is i(inifinity) equal to 21 A
At infinity, the switch is closed and steady state has been reached.  Thus the inductor behaves as a short and the capacitor behaves as an open.  The current is only determined by the power source (42 volts) and the circuit resistance (2 ohms).  The current is then 42/2 or 21 amperes.  This is the answer given in the book (under the schematic for question (c)). Don't forget that the switch remains closed after time 0.