Which of the quadratic functions below has exactly one x-intercept?

1) y = x2 − 9
2) y = x2 − 6x + 9
3) y = x2 − 5x + 6
4) y = x2 + x − 6

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?