Which relation is a function?

Two variables, x and y, vary inversely, and x=12 when y=4. what is the value of y when x=3?

Use the elimination method to solve the systems of equations.choose the correct ordered pair. 7x+4y=39

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?