Depending on your other income, this is probably correct. Line 20a reports the total amount of your social security benefits, and Line 20b reports how much of those benefits were taxable.
Generally, if your only income is Social Security, you probably don't make enough money to be required to file a federal tax return.
If your Modified Adjusted Gross Income, or MAGI (which includes Social Security) is less than the base amount for your filing status ($32,000 for Married Filing Jointly, otherwise $25,000), your Social Security isn't taxable either. Once your MAGI exceeds those amounts, at least part of your Social Security becomes taxable.
