Two known forces F1 = 3.75 N at 73.0° below the positive x-axis and F2 = 3.55 N at 33.0° above the negative x-axis are acting on an object. What is the magnitude of the third force F3 so that the acceleration of the object is zero?

when two balls of the different masses are dropped from equal height, they reach the ground at the same time .this is because they experience the same acceleration.

the acceleration is independent of mass.

thus when they are dropped they will always maintain the same velocity and travel the same distance (neglect air resistance of course) at the same time.