Can someone please explain how to do a problem like this?

Which one is true about the hexagons ?

A(1,1) b(-3,0) c(-4,-1) d(3,-2) drag it to correct position

You are standing next to a really big circular lake. you want to measure the diameter of the lake, but you don't want to have to swim across with a measuring tape! you decide to walk around the perimeter of the lake and measure its circumference, and find that it's 400\pi\text{ m}400? m. what is the diameter dd of the lake?