You need to get the denominators the same. You could multiply 2/6 by 6 on the numerator and the denominator, but that's a little awkward. It's better to divide 18/36 by 6 on the top and bottom
18 / 6 = 3
36 /6 = 6
That's as far as you need go for this question. Don't be fooled by other questions that require you to go further.
