Let the average annual growth rate of these two years be x%, and according to the preface equation 5( 1+x%)2=7.2.

That is, 1+x% = 1.2,

X 1=20，x2=-220，

After investigation, x2=-220 does not meet the meaning of the question, so x = 20.

That is, the average annual growth rate in these two years is 20%.

So the answer is: 20%