What is the measure of YB ?

Someone me i need with this stuff

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?

Find the condition that the zeros of the polynomial f(x) = x^3+3px^2+3px+r may be in a.p.