first you want to know the profits of the three ppl. so:
4800/3=1600
then u make an equation with proportions: 9x+7y+4x=4800
add the invests: 9+7+4=20x and 4800 would stay same.
then divide 20 by 4800 to get 240
now plug the investments to 240
9 times 240 is 2160
7 times 240 1680
4 times 240 is 960
last you must subtract to find x as the investments
take 1600 and subtract it by 2160 and you get x=560
$560 is your answer
hope i helped :D