What is the missing y-coordinate that would produce a slope of 3, if a line were drawn through the points left-parenthesis 4 comma 5 right-parenthesis and left-parenthesis 8 comma y right-parenthesis?

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?