Answer:
Using X = sum( x's * mass)/sum( masses)
= [(0)*(6400)+ (-9.5*1000) + (9.5)*(1000) + (9.5)*(1000)]/(6400+1000+1000+1000)
= 1.01 m
Y = sum( y's * mass)/sum( masses)
= [(0)*(6400) + (9.5)*(1000) +(-9.5)*(1000) + (-9.5)*(1000)]/ (6400+1000+1000+1000)
= -1.01 m