Answer:
x = 9
Explanation:
given two secants to a circle from an external point , then
the product of the measures of one secant's external part and that entire secant is equal to the product of the measures of the other secant's external part and that entire secant , that is
x(x + 21) = 10(10 + 17) ← distribute parenthesis
x² + 21x = 10 × 27 = 270 ( subtract 270 from both sides )
x² + 21x - 270 = 0 ← in standard form
(x + 30)(x - 9) = 0 ← in factored form
equate each factor to zero and solve for x
x + 30 = 0 ⇒ x = - 30
x - 9 = 0 ⇒ x = 9
however, x > 0 , then x = 9