Chapter 7 - Exponents and Exponential Functions - 7-3 Multipying Powers with the Same Base - Practice and Problem-Solving Exercises - Page 430: 40

$c^{-5}\times c^{11}=c^{6}$

$c^{-5}\times c^{?}=c^{6}$ To multiply powers with the same base, we add the exponents. When we add $-5$ and the second exponent, we must get $6$. The only way this is true is if the second exponent is $11$. Therefore, $c^{-5}\times c^{11}=c^{6}$