x-components:                     y-components

300∙cos(30°)  =  259.81          300∙sin(30°)  =  150.00
450∙cos(110°) = -153.91          450∙sin(110°) =  422.86
400∙cos(230°) = -257.12          400∙sin(230°) = -306.42 
--------------------------------------------------------
Total x-comp. = -151.22          Total y-comp. =  266.44

resultant magnitude =  = 306.36

tan(resultant angle) = 

Since x-comp negative and y-comp positive,
resultant is in Quadrant II, so 
Resultant angle = 119.6°

So the force that will balance the three forces is a force
equal in magnitude to their resultant and in the opposite
direction, so we add 180° to the angle and the answer is

306.36g @ 299.6°

Note: The balancing force is NOT the resultant force, but
is in the opposite direction from the resultant force.  That's
why I added 180° to the angle.  You might mention this to your 
teacher.

Edwin