>>11315777i think the idea here, though the problem is poorly written, is that the beaker is fully filled with air and the displaced water is balanced by air, which is assumed to have 0 density (also the glass is probably assumed to contribute 0 mass or volume to the system).
once you make those assumptions it is trivial. the force on the beaker (which is approximated under these assumptions to be an empty volume) is equal to the gravitational force on ball. so the force of displacing that amount of water gets equated to the gravitational force on the ball. then it's just arithmetic
