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There are two ways to do this:
(1) let u = 5x-4, and use the "chain rule", df/dx = df/du * du/dx
(2) expand the polynomial to
f(x) = 25 x^2 -40x + 16
and differentialte it term-by-term.
You will get the same answer either way.

Let's use the first way, because it is something you may need for other problems
u = 5x-4
du/dx = 5
f(u) = u^2
df/dx = df/du * du/dx
= 2 u * 5 = 10(5x -4) = 50x - 40

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