Make the denominators the same. You cannot add or subtract fractions unless the denominators are the same. The simple way to make the denominators the same is to multiply the numerator and denominator of each fraction by the denominator of the other fraction. For example, to subtract 2/5 from 3/4, multiply 2/5 by 4/4 and multiply 3/4 by 5/5. Once the denominators are the same, do the arithmetic on the numerators only and keep the denominator in the solution. So 3/4 - 2/5 = 15/20 - 8/20 = 7/20.
Change the sign of the second number and add if you're subtracting a negative fraction. Convert subtraction to addition by changing the sign of the second number and add. For example, 2/5 - (- 3/4) = 2/5 + 3/4.
Look at the signs to know what to do. If both terms are the same -- after you have changed the sign of the second number to convert subtraction to addition -- the sign of the answer is going to be the same, but you get the value by adding. For example, (-3/5) - (1/5) = (-3/5) + (-1/5) = - (3/5 + 1/5) = -4/5. Also (3/5) - (-1/5) = (3/5) + (1/5) = +(3/5 + 1/5) = 4/5. If the signs are different, the sign of the answer will be the same as the sign of the largest fraction. Always subtract the smaller fraction from the larger one -- the sign will make things right. For example, 1/5 - 3/5 = -(3/5 - 1/5) = -2/5 and 3/5 - 1/5 = +(3/5 - 1/5) = 2/5.