displacement

[Steven G]

No that's not the reason. You don't know that v=0 when t=0. The object could already be moving when the timing starts. If you were told that the object moved "from rest" you could assume v=0 when t=0.

v=0 when t=0 subbing these values into the formular for v gives c=0 ? i think

If v(t) = 4sin(3t), then surely when t=0, we have v=0
The OP clearly stated that v=0 since when t=0 was plugged into the v formula, v=0 came out.