Answer:
Step-by-step explanation:
Comment
The general formula for a tangent and secant is
tangent^2 = external segment * full length of the secant.
Givens
tangent length = x
external segment = 7
secant length = 7+ 21
Formula
x^2 = 7 * (7 + 21) Remove brackets
x^2 = 7 * 28 Combine
x^2 = 196 Take the square root of both sides
√x^2 = √196
x = 14
Answer: x = 14
