Remember that 'simplifying fractions' and 'cross-multiplying' are both really the same thing. Use them when you have a problem like this:
SOMETHING OVER SOMETHING EQUALS SOMETHING OVER SOMETHING, and you know 3 of those 4 SOMETHINGS and you need to find the missing one.
For example 3/4 = 9/?. You can use either method. You can think to yourself - "What has to be in that unknown spot?" "Three fourths of a pie is nine whats" Must be twelfths. OR you can think to yourself "3 times what" = "4 times 9". Must be 12.
If one side of that equation is a whole number, like say 6, then you can turn it into a fraction by just putting it over 1. Make 6 into 6/1.