Theorem: If a tangent and a secant meet at an external point, then the product of the length of the secant and its external segment is equal to the square length of the tangent.
If we put this into practice in this question, then it becomes:
x² = 7(21 + 7)
x² = 7 · 28
x² = 196
x = 14, because x > 0
Thus, the length of x is 14 (D)
