The resistance of a telegraph line is R = 10 , and the solenoid inductance is L

Question:

The resistance of a telegraph line is R = 10 Ω, and the solenoid inductance is L = 5 H. Assume that when sending a "dash," a voltage of 12 V is applied while the key is closed for 0.3 s. Obtain the expression for the current i(t) passing through the solenoid.