I'll add a picture so it can make more sense to you. :)
We'll use the picture below as an example okay? :)
What you want to do is multiply the numerator (top number) of the left-hand fraction by the denominator (bottom number) of the right-hand fraction. Then multiply the numerator (top number) of the right-hand fraction by the denominator (bottom number) of the left-hand fraction. Next step is to set the two products equal to each other. Then solve for the variable. If you look at the picture while reading the steps, then I think it'll make more sense.
I really hope this helps :)