1. The problem statement, all variables and given/known data 2. Relevant equations Tao = R(th) C 3. The attempt at a solution The unknown DC circuit model consists of a V(th) and R(th) Tao = (R(th) + 3000) C , total resistance of circuit is R(th) + 3000 0.004 = (R(th) + 3000) (1x10^-6) R(th) = 1 kOhm The V(th) should oppose the known source since i(t) goes to 0. Doing a KVL around the loop clockwise. -10V + 3000( I ) + I (1000) + V(th) = 0, Voltage across capacitor is 0 since at t=0 it is like steady state. also, at t=0, I = 0.001 A then solving gives V(th) = 6V I get the hunch that the working is wrong but the answer is right by coincidence. Would the working be correct? Thank you very much.