I can use the fact that the difference between two consecutive terms is $1$ to work backwards from the fifth term to the first term. If the fifth term is $11$, then the fourth term must be $10$, since I subtract $1$ to get the previous term. Similarly, the third term must be $9$, the second term must be $8$, and the first term must be $7$. Now that I have the first two terms, I can multiply them to get the product. 
The fifth term of an arithmetic sequence is $11$. If the difference between two consecutive terms is $1$, what is the product of the first two terms?