The 'D', to the left of the bust, is a mint mark. The coin was produced at the Denver Mint. Welcome to the forum.
First, welcome to the neighborhood, Christina! I assume that 1976 was just a typo, and you meant 1979. The Susan B. Anthony Dollar was only struck for three years, 1979-1981, and then again in 1999 as an interim dollar coin until the Sacagawea "Golden Dollar" was ready for circulation in 2000. The D-shaped mark to the right of Susie B's face is post-mint damage (PMD) and detracts from the collector interest in the coin. Chris