How do you find the derivative of square root of five /t^5?

how do you find the derivative of square root of five /t^5

I know that f=a t^-n
f'= -an t^(-n-1)

and i tried putting -25/{[2sqrt(5/t^5)]/(t^4)}

Goodness. sqrt(5t^-.5)

derivative is -.5sqrt 5t^-1.5