Resolve the 400N force into its horizontal and vertical components, giving 400cos60 for the vertical component and 400sin60 for the horizontal component. Now 400cos60 is equal to 200N which cancels out with the force being applied upward.
Now since we know the NET force is zero, for this to occur the horizontal component of the 400N force must be canceled out by the third force, i.e. the third force must cancel 400sin60. Thus the magnitude is 400sin60 = 346.41N to the right
Try and apply the same concept to Part (b), resolve the forces which aren't vertical or horizontal into their separate horizontal and vertical components.
If you still don't get part (b) OR (a) just say so