A particle of mass 5.0 kg is under the influence of 2 forces. If the particle is accelerating at a rate of +4.0 m/s2, and the first force is +25 N, what is the second force?

The net force on the particle is

M*a = 20 N

If the first force is 25 N, the second must be -5 N, if forces can act along one axis only.

In a two dimensional situation, other answers are possible.