Three objects of equal mass

m$1=$ m$2=$ m$3=2\mathrm{.}60$ kg

are located on the vertices of an equilateral triangle of length

d $=3\mathrm{.}30$ m$.$

A fourth mass

m$4=15\mathrm{.}5$ kg

is at the center of the triangle. What is the net gravitational force on the mass

m$1$

that is at the origin? Express your answer in vector form.