The resultant force is the overall force: say you had 10N acting left and 7N acting right on an object, its resultant force would be 3N left as they cancel out. The same goes when these forces are at different angles.
Think about if you had 3N pushing up and 4N pushing left on a particle - these two lines drawn out would be perpendicular. You'd notice the particle moves towards the top left, as this is the "net force", i.e. the resultant force.
Like vectors in maths, a translation of (3,4) could be thought of as moving 3 right and 4 up, making an L shape, or it could be thought of as a direct diagonal line. This diagonal line is the resultant - hence we can make a right angled triangle and use trigonometry, by treating the forces as lengths. We end up with a magnitude of 5N (see
a and
b)
Once you have the magnitude, you need to find an angle. Here we can use Cos = Adjacent/Hypotenuse to get 36.9 degrees (see
c)
For your question, you have 4 different forces. To deal with these, simply find the resultant of one pair, then the resultant of the other pair, then finally the resultant of the two new forces. If you struggle with it, let me know!