Answer:
x = 15
Explanation:
the segment inside the triangle is an angle bisector and divides the side opposite the bisected angle into segments that are proportional to the other two sides, that is
=
( cross- multiply )
10x = 25(21 - x)
10x = 525 - 25x ( add 25x to both sides )
35x = 525 ( divide both sides by 35 )
x = 15