How can I calculated preferentially taxed income? I can not match taxable income with schedule.

As I know, preferential tax rate of 0%, 15%, or 20% for stock dividend and long-term capital gain.

I have got solution of this question but I am still confused.

In this example,the solution part that confuses me, is 9,000 tax (4000*0% + 5000*15%)=750.

My questions:

In this question solution given below:

1.Why is 9,000 divided into 4000 and 5000 ?

2. 4,000 match 0%, and 5000 match 15%.

However, in MFJ, 0~9275 taxed at 10% ordinary rate and 0% at preferential rate. 4,000 and 5,000 are less than 9,000,so both of them are taxed at 0% preferential rate.

a. What is the Comers' tax liability for 2016 if they report the following capital gains and losses for the year? Short-term capital gains Short-term capital losses Long-term capital gains Long-term capital losses S 9,000 (2,000) 15,000 6,000) Total tax liabilit 10,518 Explanatlon: a. Short-term capital gains Short-term capital losses Net short-term capital gain S 9,000 (2,000) 7,000 Long-term capital gains Long-term capital losses Net long-term capital gains $15,000 6,000) $ 9,000 Salary Net short-term capital gain Net long-term capital gain AGI Standard deduction Personal exemption Taxable income Less preferentially taxed income Income taxed at ordinary rates Income subject to capital gains rates S 85,000 7.000 9,000 $101,000 (12,600) (8.100 S 80,300 (9,000) $ 71,300 tax $1,855+ $7,913=$9,768 $ 9,000 tax ($4,000 0% + $5,000 x 15%-$750 Total tax liability = $9,768 + $750 $10,518