I’m very sorry @AriHoresh I couldn’t find the “info” thing :((
I recently studied this subject myself so it would be best if someone else reviews it to just in case 
So resultant force is the total force acting on a body. In the IMAT level we are considering only 2 directions: Up&down (Y- axis) and Left&right (X-axis). lets review the forces in the Y axis first:
we have the gravitational force witch is 5kN, and the normal force witch is also 5KN exactly. the normal force and the gravitation force cancel each other out. the force on the Y axis is 0.
If you have trouble understanding this concept you can read this link
continuing on the X - axis:
to the right we see a force of 1500N (probably the engine of the train) to the left we see a force of 400N (probably friction). But you are told to consider the left direction as negative, so -400N.
lets calculate the force on X axis = 1500N - 400N = 1100N
we haven’t finished yet now we need to get the magnitude of the resultant force:
And that the final answer
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