adding and subtracting rational expressions

When adding or subtracting rational expression you always have to see that the denominators are the same. For example, when subtracting 6/5d and 2/3d you have to find a common denominator. To do this you can multiply each by each other. For example 5d and 3d have the same variable so you can leave that alone but you have to change the number, so you would multiply 5 times 3 to get a common denominator. So the common denominator would be 15d. But what ever you do to the bottom you have to do to the top. So the new expression would be 18/15d-10/15d. lastly you just subtract the numerators and leave the denominators alone. So your final answer would be 8/15d.