7(a-b)-8(a-2b)

To simplify the expression 7(a-b)-8(a-2b), we can use the distributive property and combine like terms.

Step 1: Distribute the first term (7) to both terms inside the parentheses.
7(a-b) becomes 7a-7b.

Step 2: Distribute the second term (-8) to both terms inside the parentheses.
-8(a-2b) becomes -8a+16b.

Now we have the simplified expression:
7a-7b-8a+16b.

Step 3: Combine like terms.
Combine the terms with 'a' together and the terms with 'b' together.
7a - 8a = -1a = -a
-7b + 16b = 9b

The final simplified expression is -a + 9b.