Thank you for the opportunity to help you with your question!
c = 2
To do this problem, you have to understand exponent rules:
(x^3)^c = x^(3c)
When you have an exponent to an exponent, you multiply the exponents together. If you look at the right side of the equation, you can see that it is the square of everything on the left hand side of the equation, so 'c' must equal '2'.
Please let me know if you need any clarification. I'm always happy to answer your questions.