You can multiply both sides by a number that will get rid of the denominators. For ex. you can multiply the first equation by 12 because 4 and 3 both go into 12 evenly. MAKE SURE YOU MULTIPLY THE OTHER SIDE BY 12 AS WELL! Therefore, 12(x/4) - 12(y/3) = 12(1), or 3x - 4y = 12