A problem with forces in two dimensions

Fred, Betsy and Joe all pull a tire. The tire's mass is M = 75 kg. The force of each person is

Fred: 300 N to the East
Betsy: 340 N to the North
Joe: 400 N at an angle theta South of West

The net result is that the tire accelerates slowly to the North, at a = 1.0 m/s².

What is the angle theta?

   force        x            y
  --------------------------------
           |          |         
           |          |         
           |          |         
           |          |         
           |          |         
           |          |         
           |          |         
           |          |         
  --------------------------------
   total   |          |
  --------------------------------



                x           y
  --------------------------------
   accel   |          |
           |          |
  --------------------------------