Been some time I haven't touched mechanics. Haha:)) For part a, resolve both forces by splitting them into vertical and horizontal components. Apply pythagoras theorem to find resultant force.
As for part B, resultant force must be equals to zero. The third person must exert the same amount of force in the opposite direction as force is a vector quantity. The angle is used to determine the direction and is found by using trigonometry. Hope it helps!!