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This is what I've started with but I'm not sure how to proceed and if I even started correctly:

f'(x)(-1/2)x^(-1/2)=1/2sqrt(x)

f"(x)=(-1/4)x^(-3/2)=-1/4x^3/2

f"'(x)=(3/8)x^(-3)=3/8sqrt(x)

f""(x)=(-9/8)x^(-4)

and then i just plug 4 in for x

any explanation toward the correct answer would be great,thanks