Maths problem, how to do (b), thanks?

(b)
[First analyze the statement to be proved.]
BE² = AC × DC - EC²
⇔ BE² + EC² = AC × DC
Here the left-hand side is actually BC² by Pythagoras theorem.

Therefore, we are actually asked to prove
EC² = AC × DC,
then it is a clear similar triangle setting.

[The above is the thinking process. Then we proceed to show the work.]

From (a), it is proved that ΔABC ~ ΔBDC.
Then,
AC/BC = BC/DC (corr. sides, ~Δs)
BC² = AC × DC
BE² + EC² = AC × DC (Pyth. thm.)
BE² = AC × DC - EC²