How do I find the reultant force acting on an object sitting on a slope?

Firstly, you should consider what forces are acting on the object and in what direction. It helps to draw a large diagram to label. In most questions, the forces should be told to you. It helps to keep in mind the formula: F = m a .
Next, you should resolve the forces into their horizontal and vertical components. You can find the vertical and horizontal components of a force f on a slope of angle a' by using the formulas (f)sin(a) for the vertical component and (f)cos(a) for the horizontal component.
Once all of these are found, you should add together all of the horizontal components and all of the vertical components, keeping in mind which are negative and which are positive.
You should then combine the resultant vertical force F_v and the reultant horizontal force F_h using the pythagorean theorem: F² = F_v² + F_h² .
You can then find the angle at which this force is acting using the trigonometric equation: tan(a) = F_v / F_h .