Cody W. answered 02/24/23
Tutor
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MIT Graduate - Contest Math Instructor at Art of Problem Solving
This question is testing your knowledge of the chain rule.
H(x) = f(x2 - 9)
This means that H'(x) = f'(x2 - 9) * (x2 - 9)'. This is coming from the chain rule
H'(x) = f'(x2 - 9) * 2x
H'(4) = f'(16-9)* 2(4) = f'(7)*8
H'(4) = -7*8 = -56