.. a=1 so

.. expand this part

Here you can compare

--- 1

--- 2

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Now make use of the factor and remainder theorem

You know one of the factors is (x-1) so

f(1)=0

--- 3

Then it will give a remainder of 7 when divided by (x-2)

f(2)=7

--- 4

4-3

Now look back . From 2 above , --6

(Replace the c with -3b)

Multiply 1 by 3 ... -- 5

6-5 i will leave this for you .