Use part I of the Fundamental Theorem of Calculus to find the derivative of \[ \begin{array}{l} f(x)=\int_{3}^{x}\left(\frac{1}{3} t^{2}-1\right)^{7} d t \\ f^{\prime}(x)= \end{array} \]

Step 1 ：Given the function \(f(x) = \int_{3}^{x} (\frac{1}{3t^2} - 1)^7 dt\)

Step 2 ：According to the Fundamental Theorem of Calculus Part I, the derivative of this function is simply the integrand evaluated at x

Step 3 ：So, we have \(f'(x) = (\frac{1}{3x^2} - 1)^7\)

Step 4 ：Therefore, the derivative of the function is \(\boxed{(\frac{1}{3x^2} - 1)^7}\)