4). A particle of mass m kg is at rest on a rough plane inclined at an angle above the horizontal. Force X N acts on the particle up the greatest slope of the plane, as shown in the diagram. Find the magnitude and direction of the frictional force when:

a). X = 10, m = 1

b). X = 10, m = 2

I don't really have any idea what to do. The answer for part 1 is 5.1N, and part 2 is 0.2N. Can you help?