This is a really basic question...how do you know whether to divide or multiply the numerator / denominator when you're simplifying an equation with fractions?

For instance, if you're solving for X:

x/3 = 6

Do you divide the 6 by 3 (x = 6/3 = 2) or multiply it by 3 (x=18)?

Is there a rule you can follow (i.e. you multiply when you're moving the number in the denominator, and divide when you're moving the number in the numerator)?

Thanks,
David