If they are coplanar then all three lie in the same plane. That means 2D
The resultant force is the sum of the component forces of vectors. So if you have a vector of magnitude 8 going left and another vector of 6 going right, the resultant will be 2 (8 - 6). Its -6 because it is going in the opposite direction.
Now with vectors at an angle you have to first find the vertical and horizontal components. You'll have pythagora's theorem. sin=o/h , cos=a/h , tan=o/a. Easy way to remember is SOH CAH TOA.
Once you have the vertical and horizontal components, add them together and use pythagoras theorem again to find the magnitude.
To find the direction, you can use TOA. You need have the vertical and horizontal components. tan@ = o/a
changes to @ = tan-1(o/a) , which is inverse tan with -1 at the top right of tan (you can find it on your calculator.
If you didn't understand it please say. And remember, google is your friend
This site will help if you go through it
http://s-cool.co.uk/alevel/maths/vectors-lines-and-planes.html