There are 3 vertical forces here, of which we know only one (the weight of the plank). I hope you realize that if you take moments about the pivot point you will get an equation that does not contain one of the unknowns(the upward force through the pivot point). The equation will contain the only other unknown, which is the weight of the mass added to the short end.

First, add to the diagram the vectors representing the two downward forces. Also indicate their distances from the pivot point.