Simple: Addition of the multiplcations of each of the forces by each associated distance from the fulcrum on each side will equal the other if the plank is in balance - if not, then the difference between the two sides will give the amount and the direction of the resultant turning moment.

That should be enough help to get you started on your homework! First History, now maths... Max, when did you advertise the Mudcat as a Homework help guide, or is this part of the new Megolomanic (sorry, Mudcat!) Fund Raising Scheme?

Robin