Find the value of a so that the function f(x) = xe^(ax) has critical point at x = 9

Question

I found that the derivative is axe^(ax)+e^(ax)

But I am not sure where to go from here?

Answer 1

e^(ax) (ax+1) = 0 for derivative = 0
so
a = -1/x
a = -1/9