Final answer:
The simplified expression is 6x - 18.
Step-by-step explanation:
The expression 6x - 18 can be simplified by combining like terms. In this case, the terms 6x and -18 have no like terms to combine with, so the simplified expression is 6x - 18.
The answer is none of the given options: a. X59, b. XS2, c. X845, or d. X$7.5.