OpenSAT

Draw a radius from the center of the circle to each endpoint of the chord. This creates an isosceles triangle with the chord as the base. Draw a perpendicular segment from the center of the circle to the chord, bisecting the chord. This forms a right triangle with legs of length 4 and hypotenuse of length 6. Use the Pythagorean theorem to find the length of the other leg, the distance from the center of the circle to the chord: $4^2 + x^2 = 6^2$, so $x = \sqrt{20}$.