Solution: According to the conditions, △EFG∽△GHD can be obtained.

Get: ef: Hg = fg: HD。

And EF=a-b, FG=b, HG=b-c, HD=c,

Then (a-b): (b-c) = b: c,

Then you get: B2 = AC.

The relationship between a, b and c is B2 = AC = 4.

So b = 2.

So the answer is: 2.