Answer:
30
Step-by-step explanation:
triangle ABC is proportional to triangle BCD so
hypothenuse of ABC/ ajdacent of ABC = hypotrnus BCD/ ajdacent of BCD
in short
AC/BC = BC/DC
10/x = x/3
10 . 3 = x²
x² = 30
x = √30
so the question mark is 30
