This asks for the middle 97% or 0.97 of the data.  Half of 

the middle 0.97 is to the right of z=0, and the other part

is to the left of z=0 on the graph  Half of 0.97 is 0.485.

We look through the BODY of the normal table at this site

http://business.statistics.sweb.cz/normal.jpg  

which should be just like the normal table in your book.

You will find the listing 0.4850 near the bottom of the table

on the right.  You will notice at the far left of the row

in which you find 0.4850 is the z value 2.1 and at the top of 

the column in which you find 0.4850 you'll find the value 0.07. 

You add those two numbers together 2.1+0.07 and get z=2.17. 

You know by symmetry of the normal curve, that the other half of 

the middle 97% is from z=-2.17 to z=0.  So the critical values

are z=-2.17 and z=2.17

Edwin