How do you solve: 11a=18a+21 ?

11a = 18a + 21
11a - 18a = - 18a + 21
-7a = 21
7a = -21
a = -21/ 7
a = -3

Ok, the points is to isolate the variable "a". First, you subtract 18a from both sides to remove the 18a from the right side, ending up with -7a = 21. Dividing both sides by -7, you will end up with a = -3. a = -3

Yes you do, so £1973.09 - £1473.50 = £499.59 so tim overpaid £499.59 .hope this ; )