Two people are pulling on a mull (opposite directions). Force 1 is 120 Newtons at 60 degrees. Force 2 is 80 Newtons at 75 degrees. How do I find the single force equivalent to the two forces indicated? If a third person was pulling on the mule, what force would have to be exerted on the mule to make the resultant force equal to zero?

1. You have to draw a sketch.
2. Break the forces into their corresponding components.

A force of 4N at 45 degrees to the horizontal is broken into a vertical force of 4\sqrt2 N and a horizontal right force of 4\sqrt2 N. If you combine those two forces by pythagoras theorem, you'll have the 4 N back.

Now, if you want the resultant to be zero, you have to cancel all the resultant components, that is providing the equal and opposite force.

I hope that it helped. Post your answer to see if you understood well, or perhaps you didn't understand something, OK? :)