Find the Thevenin's equivalent of a circuit between nodes A and B
I’m using an iPhone to answer so pardon the math. The reason Vth=-20*i(t)*25 and not Vth= 20*i(t)*25 where * is just multiplication is due to the simple rule of passive sign convention. When current flows   through the positive side of terminal of passive component it establishes positive voltage and when current flows into negative terminal it produces negative voltage. 

In your circuit current from the voltage controlled current source enters into the negative side of 25ohm. Producing a voltage of V(t)=-20*i(t)*25.