Bill made $1000 of taxable income.
The tax rate is:
15% for the first $7500 plus 18% for any amount above $7500.
Please check that you typed the correct taxable income, since you typed $1000 (one thousand) and there is no quantity above $7500 in such case.
So if the income is in truth $1000, then all we need to apply is the 15% of the income, because $1000 doesn't go above the limit of $7500.
This is then:
15% of $1000 = 0.15 * 1000 = $150
So the taxes Bill needs to pay is $150 (one hundred fifty dollars)