gravitational potential energy (aka GPE or Ug) = mass*gravitational force*height
GPE=5*10*3= 150 J
Side-note: If you were to use a real-life scenario/accurate gravitational force, you would use 9.8 N.
So the most “accurate” equation would be 5*9.8*3=147 J, which is already roughly near 150 J.