Capital gain net income for line 2a of the Return of Excise Tax on Undistributed Income is calculated by subtracting the total capital losses from the total capital gains reported on the taxpayer's income tax return for the taxable year. Capital gains and losses are reported on Schedule D (Form 1040), Capital Gains and Losses. It is important to note that only net capital gains or losses are reported on the Return of Excise Tax on Undistributed Income, not the individual gains or losses themselves.