updating previous question so easier to understand

im using chain rule to answer this question:

f(x) = -5 sqrt(x^2 - 4x + 1)
what is f'(x)?

tried to use calculator to get the answer to base my answer off of but this is a nonreal number. does this mean i will have to use imaginary numbers? please help

f'(x)=[-5* 1/2]*[1/sqrt(x^2-4x+1)] * (2x-4)