Okay first you should look at the denominator and if the denominator are the same you could subtract easily but in this case the denominator are different so you see which denominator is the biggest one and try to chose the small one. So basically what four times what equals to 16 given the 4 and then you multiply the fraction 3/4 by 4 and giving 12/16 and then you subtract and giving the answer 3/16