Let $\displaystyle f(x; \theta) = \theta e^{-\theta x}$. Find the complete sufficient statistic for theta.

From what I understand, I have write the PDF in the "form" of the Regulat Exponential Class/Family which has the PDF of $\displaystyle f(x; \theta) = e^{p(\theta )K(x) + S(X) + q(x)}$

$\displaystyle f(x;\theta) = e^{-\theta x + log \theta} $

so I know $\displaystyle p( \theta) = - \theta$, $\displaystyle K(x) = x$ S(x) = 0 and $\displaystyle q( \theta ) = log \theta$

I don't understand my textbook's explaination of how to find the complete sufficient statistic, would someone please explain the next step I have to take?

Thank you.