In the circuit, 1&2 are joined for long time. After disconnecting 1&2, 2&3 are connected. The charge on the capacitor when the current in the inductor (ideal) decreases by 3A is [nearly] 
 

When 1&2 are closed for long time $i_{max}=\frac{5}{1}=5amp$  through inductor .
When 2&3 are joined, by law of conservation of energy $\frac{1}{2}L{i}_{\max }^2=\frac{1}{2}L{i}^2+\frac{q^2}{2C}$
$\frac{1}{2}\left(2\times {10}^{-3}\right)\times 25=\frac{1}{2}\left(2\times {10}^{-3}\right)\times 4+\frac{q^2}{2\times 4\times {10}^{-3}}$$q=\sqrt{168}\times {10}^{-3}\approx 13\times {10}^{-3}C$