While two forces act on it, a particle of mass m=3.2kg is it to move continuously with velocity (3m/s)i - (4m/s)j . one of the forces is f1 = (2n)i + (-6n)j . what is the other force?

Mmm tricky.
Since the velocity is constant, I'm going to assume there is no acceleration in any direction. This means there is no net force in the I or J forection!

Since there are 2 forces, both must be equal and opposite in direction to perfectly cancel each other out.

So the opposite of F1 is (-2N)I + (6N)J!

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