Can someone explain how to solve this?

21.5

Step-by-step explanation:

x is the radius of the circle.

If we draw a line from the circle's center to either end of the horizontal line, that line will also be a radius, so it will have length x.  This forms a right triangle with side lengths 10 and 19 and hypotenuse x.

Using Pythagorean Theorem:

c² = a² + b²

x² = 10² + 19²

x² = 461

x ≈ 21.5