My friend really needs help with solving this problem

In the polynomial below, what number should replace the question mark to produce a difference of squares? x2 + ? x - 36

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?

Given δjkl : δxyz, find x. a)10 b)12 c)16 d)20