The reasons are as follows: set the flagpole height ab = X.

If F is passed, FG⊥AB is in G and CE is in H (as shown in the figure).

∴△AGF∽△EHF.

∫FD = 1.5，GF=27+3=30，HF=3，

∴eh=3.5﹣ 1.5=2,ag=x﹣ 1.5.

From △AGF∽△EHF, we get: =,

That is =,

∴x﹣ 1.5=20，

Solution: x=2 1.5 (m).

Answer: The height of the flagpole is 21.5m. 。