Answer
X=9
Work Step by Step
We know that:
$$ AB+BC=AC$$
Plugging in known values, it follows:
$$ 21=x+x+3 \\ 21=2x+3 \\ 18=2x \\ x=9$$
Elementary Geometry for College Students (7th Edition)
by Alexander, Daniel C.; Koeberlein, Geralyn M.
Published by
Cengage
ISBN 10:
978-1-337-61408-5
ISBN 13:
978-1-33761-408-5
Chapter P - Preliminary Concepts - P.3 - Informal Geometry and Measurement - Exercises P.3 - Page 24: 37
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