confused on how to calculate self employment tax on a tax return 1040 for 2017.

Schedule C (this is what i have)

Gross Income 110,000

Total expenses before expenses for business use of home. Add lines 8 through 27a . 31,400

Tentative profit or (loss). Subtract line 28 from line 7 . . . . . . . . . . . 78,600 (110k-31.4k)

##taxquestion