Mass m$1=141\mathrm{.}2$ kg is located at the origin of some coordinate system. A distance $15\mathrm{.}3$ m away is mass m$2=17\mathrm{.}7$ kg$.$  How far from mass $1$ should mass m$3=78$ kg be placed if the net gravitational force on mass $3$ from the other two is zero? First, use a free body diagram to convince yourself mass $3$ must be on the line joining masses $1$ and $2$