"A number" signifies the need for a variable, in this case, n. The keyword increased shows that the operation is addition. So put it together and you get n+8.
substitute 3 for x in the equation and you get 730 and then substitute 1 for x and you get 28. subtact 28 from 730 and you get 702. then do "b-a" so 3-1 and you get 2. finally, divide 702 by 2 and you get 351.