"collinear" means along the same line; or if it refers to points, then it means the points lie along a straight line.
If you sum vectors A and B you get the same result as if you sum vectors B and A.
The picture with the chains means that the forces, F1 and F2, in two of the chains are added (vectorially) to give a resultant vector (F1+F2). This is then added (vectorially) to the force F3 to give an overall resultant force FR.
Adding vectorially means you add the x-component values together, and add the y-component values together.
In the s***w eye you need to resolve F1 into it's components (F1*cos(theta1), F1sin(theta1)) and F2 into its components (F2cos(theta2), F2sin(theta2)), (where the thetas are angles between the vectors and the x-axis) and get the components of the resultant from
(FRx, FRy) = (F1*cos(theta1)+F2cos(theta2), F1sin(theta1)+F2sin(theta2))
The magnitude of the resultant is given by √( FRx2 + FRy2) and its angle to the x-axis by tan-1(FRy/FRx)