Although Desmos was the clear enabler of the correct solution for the vast majority of students, I start my solution by looking at the general equation of a circle, which is based on the Pythagorean Theorem.  Next, we look at how Desmos easily finds our answer, as long as you have at least one finger to type each equation.  Challenge: Can you figure out how to make an interactive radius in Desmos?  Prize: A red 1972 Ford Pinto Runabout with original white vinyl interior.  Duration: 15:43