Francine is standing on two bathroom scales - one foot on each scale. The left scale reads 500 N, and the right scale reads 400 N. Find her mass.

Please help or just show the equation to solve

Gravity, from the center of mass of the box, extending downward normal force, from the floor under the box, extending upward, equal to gravity push force, acting at a point on the box, extending in the direction of motion friction, acting at any point on the box directly opposite the push, extending directly opposite to the motion

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