Math Help - How does this work, finding the 3rd one?? stuck

3 forces act on a moving object. 1 force has a magnitude of 80 N and is directed due north. The other has a magnitude of 60 N and is directed due west. What must be the magnitude and direction of the 3rd force, such that the object continues to move with constant velocity??

I have no clue where to start. Can someone get me started. Thanks.
Do I have to use 2d kinematic formula for this??
thanks

Object move with constant velocity ---> The sum of the forces acting on that object must be zero.