When solving the problem, you need to take the sides of the problem individually. According to PEMDAS, we need to do the division first.
(6÷2) - (18÷3)
2 goes into 6 3 times, so the left side's answer is 3.
3 goes into 18 6 times, so the right side's answer is 6.
Now, you have 3-6.
And using our subtraction skills, the answer will be -3.