Question 21912
the least common denominator would be (a)(2b).  if you were to work the problem the best advice for thing like this I can give is to work it in the following manner.

5/a-7/2b

((a)(2b)* 5/a)- (7/2b *(a)(2b))  in the first number the a's would cancel 
                                 in the second the 2b would cancel
5a-14b                I multiplied the denomonator that was left after the cancelling by the number.  do you see what I did?  This tip works great for solving problems with variables, large numbers, ect...