Don't panic. Please refer to the figure attached.
y is the radius; I drew a couple of more there.
We see we get a right triangle with hypotenuse y, one leg 5, and the other leg 9/2, as the radius perpendicular to a chord bisects the chord.
We could write the Pythagorean Theorem down and solve for y but don't have to. We're after x. x is the leg of a right triangle with other leg 5 and hypotenuse y. It's the same as the other triangle. So x is the other leg, 9/2.
Answer: 9/2
