The answer would be 72. Let's look at the denominators to solve the problem. First, let's see if the denominators are equivalent or related in some type of way. 84 and 7 are related because 84 divided by 7 equals 12. Now, according to the law of proportion, the left must be equal to the right, and that concludes that in the end, the the two fractions must be equivalent. Since you have to multiply 7 by 12 to get the output of 84, multiply the top number 6 by 12 to get the answer of 72. Why I chose 72 instead of -72 is because there is already a negative sign on the right side as on the left. Another way you can do this is by cross-multiplying.
