Problem 1
We replace every copy of x with 4c and simplify like so:
f(x) = 8 - 5x
f(4c) = 8 - 5*4c
f(4c) = 8 - 20c is the answer
This is equivalent to -20c+8.
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Problem 2
Same idea as before, but this time we'll plug in x = 2-k
Each x gets replaced with (2-k)
f(x) = 8 - 5x
f(2-k) = 8 - 5(2-k)
f(2-k) = 8 - 10 + 5k
f(2-k) = -2 + 5k
This is the same as saying 5k - 2.
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Problem 3
The steps are similar to earlier.
f(x) = 8 - 5x
f(4p+3) = 8 - 5(4p+3)
f(4p+3) = 8 - 20p - 15
f(4p+3) = -7-20p
This is the same as writing -20p - 7.