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Explanation
The idea is to plug in x = 2b, which means we replace every x with 2b and simplify
f(x) = 5(x-4) + 9
f(2b) = 5(2b-4) + 9 .... every x replaced with 2b
f(2b) = 5(2b)+5(-4)+9 ... distribute
f(2b) = 10b-20+9
f(2b) = 10b-11
The 10b and -11 are not like terms, so we cannot combine them. We leave it as the last line shows.
