Ok you divide the force of friction by the applied force (11250/15000) and that gives the coefficient of friction, coefficient of friction is always between 0 and 1, when divided out you get 0.75 set that equal to the force of friction equation (Ff=(g)(m)(coefficient of friction)) so you get 11250=9.81(0.75)m and then you simplify to 11250/(9.81)(0.75) and you get 1529 kg with sig figs I’d say you can do 1530 kg, by the way the g is for acceleration produced by gravity which is 9.81 on earth.