Each city district will have a stone pillar around the center of the district. If you have cleaned up all the trash in the area then you can select the stone pillar, and you will find that you can unlock statues to put on display.

Periodically these statues can be tapped to add an amount of net worth to your total net worth score equal to the value of all the decorations you currently own, and each additional statue you have in the city will add a bonus!

Collecting this net worth from the statues will complete your quest for you.