If there’s an amount in Box 2a of your Form CSA-1099R, the amount in box 9b isn’t needed for your tax return.

The Office of Personnel Management has probably already computed the taxable amount of your annuity in Box 2a, Taxable amount.  If Box 2a was blank, then you’d need to use the amount in box 9b, Total Employee Contributions, to figure the taxable amount of your pension.