Question 290094
Cross multiplication used to be frowned on by teachers because you aren't obviously doing the same thing to both sides.

3/4=x/7
The proper way is to 
multiply both sides by 4
3*4/4=x*4/7
3=4x/7
then multiply both sides by 7
3*7=4x*7/7
simplify
3*7=4x
but with cross multiplication we skip the step of showing that we multiply one side by each number.
3/4=x/7
we jump right to the result
3*7=4x
It is shorter and therefore preferred by students.
However there is more chance of errors and not really understanding what you are doing and why.