find the derivative of the function. use chain rule.

There is a formula that says: d(a^u)/dx = a^u·ln(a)·du/dx

(If you don't have this formula, then we'll have to go back to more basic formulas.)

(Assumes: a>0 and u is a differentiable function of x. The inclusion of du/dx makes it a chain rule problem.)

For your problem, a = 5, u = -1/x.   

--> a^u = 5^(-1/x) 

--> ln(a) = ln(5)

--> du/dx = d(-1/x)/dx = d(-x^-1)/dx = x^-2 

y' = 5^(-1/x) · ln(5) · (x^-2)

Thank you that make sense now. I apreciate that. :)