Answer:
f'(x)=-(x+42)/5
Explanation:
Rewrite f(x) as y:
y=-5x-42
Flip x and y and solve for y:
x=-5y-42
x+42=-5y
(x+42)/-5=y
So the inverse function is f'(x)=-(x+42)/5 (note that f' means inverse)
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