Usually when you see f(x) or as yours is f(6) f with parenthesis is just a fancy way of saying y= so basically you plug six in for the x values of your question
then it will be 3/4*5/3 which then equals 15/12 after that you would divide by 3 cause it is the only number that would go in evenly then the answer will be 5/4 in the simplest form