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
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