Step #1:
Remind yourself of PEMDAS.
The 'MD' part reminds you that you're supposed to
take care of any multiplication or division before you
worry about the 'AS' ... the addition and subtraction.
Step #2:
Look for multiplication or division.
Do you see any ?
I see -12 ÷ 18 and that's - 2/3 .
Step #3:
NOW do the subtraction.
- 2/3 - 6 = - (6 and 2/3) .
