Chapter 7 - Exponents and Exponential Functions - 7-3 Multipying Powers with the Same Base - Practice and Problem-Solving Exercises - Page 430: 37

$5^2\times5^9=5^{11}$

$5^2\times5^?=5^{11}$ To multiply powers with the same base, we add the exponents. When we add $2$ and the second exponent, we must get $11$. The only way this is true is if the second exponent is $9$. Therefore, $5^2\times5^9=5^{11}$