Think of the letter 'a' to represent 1 apple.
a = 1a = 1 apple
2a = 2 apples
3a = 3 apples
and so on
Adding 7a with 4a means we have 7 apples and we add on 4 more. We'll have 7+4 = 11 apples total.
Algebraically we write 7a+4a = 11a to do this using shorthand.
The 9-7 portion is calculated like you'd expect and it simplifies to 2.
------------
Here's what the step by step picture looks like
(7a+9) + (4a-7)
7a + 9 + 4a - 7
(7a+4a) + (9-7)
11a + 2
