Resolve each vector into its x and y components. For the 6N force at 135°, the components are:
F = 6·cos (135°)i + 6·sin(135°)j where i and j are the unit vectors in the x and y directions respectively
To get the net force, add all of the x components together then all of the y-components together. The equilibrant is equal and opposite to the net force (change the signs on the x and y components of the net force).
To do it graphically, put the non-arrow end of each vector at the arrow end of the previous vector. You can add them in any order. The resultant vector goes from the non-arrow end of the first vector to the arrow end of the last one. The equilibrant is equal in length and opposite in direction.