When the beam is in equilibrium, you can sum moments of a force about any point and set the sum equal to zero, but you must compute the moments of each force about that same chosen point, and then watch your plus and minus signs (clockwise vs. counterclockwise). Although you can choose any point, it is convenient to choose a point on the beam at one of the reaction supports. Try again to sum moments about R_a, and watch your plus and minus signs. Note from the symmetry of the loading and beam, the values of the end reactions, as you gain more experrence, should pop right out at you.
