How do you know that moving the line down is a valid move?

Module 2 Week 3 Day 11 Challenge Explanation Part 2
How do you know that moving the line down is a valid move?

Hi bravekiwi,
Just to clarify your question, when you say "line down," I assume you are referring to the line from the center of each circle to where the line touches each circle, correct? If they, your question is how do we know we can draw such a line, that's a great question!
One of the nice properties of a circle is that for any tangent line (that is, a line which touches the circle at exactly one point), it makes a 90degree angle with the radius. So we can say that for both of those circles, a line from the center to the point of contact will make a 90degree angle.
I hope that's helpful. Let us know if you need more clarification!
Happy Learning,
Alex F.