12 - 6/(6)(5) - 30

= 12 - 1*5 - 30 (multiplication and division are performed in order left to right)

= 12 - 5 - 30 (all multiplication is performed before addition and subtraction)

= 7 - 30 (addition and subtraction are performed left to right)

= -23

_____

Parentheses alter the order of operations, because the contents of parentheses are evaluated first. If the above expression is not what you meant, you need to write it using parentheses where needed to make it be what you meant.

= 12 - 1*5 - 30 (multiplication and division are performed in order left to right)

= 12 - 5 - 30 (all multiplication is performed before addition and subtraction)

= 7 - 30 (addition and subtraction are performed left to right)

= -23

_____

Parentheses alter the order of operations, because the contents of parentheses are evaluated first. If the above expression is not what you meant, you need to write it using parentheses where needed to make it be what you meant.