Comment
The general rule for this question is that the distance from the point to the nearest edge of the circle times the full length of the secant to the furthest edge is a constant.
Givens
Upper secant has two lengths 6 and (6 + 9) which is 15
The lower secant is 3 and (x + 3)
6*15 = k
3*(x + 3) = k
Therefore
90 = 3(x + 3) Remove the brackets.
90 = 3x + 9 Subtract 9 from both sides.
90 - 9 = 3x
81 = 3x Divide by 3
x = 81/3
x = 27 <<<< Answer