since the box will move only in the horizontal direction and no net vertical motion then the force for the horizontal motion is equal to fcos θ. then this force is equal to the normal force and friction force exerted by the floor.fcos θ = us (Mg + fsin θ)fcos θ = us Mg + us fsin θfcos θ - us fsin θ = us Mgf(cos θ - us sin θ) = us mgf = us Mg/ (cos θ - us sin θ)