Help. ​

"x" will be a radius and it will be the same length as the hypotenuse formed by the right triangle with legs = 7   and (1/2) of the chord length  = (1/2)(12)  = 6

So......

x = sqrt ( 7^2 + 6^2)  =  sqrt ( 49 + 36)  =  sqrt ( 85) ≈  9.2