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**Roam** $\displaystyle a_k$ represents "the current value" and $\displaystyle a_{k+1}$ represents the value that comes after your current value. So in this problem you start with $\displaystyle a_1 = 28$, your initial value is 28. Now you want to find the thing that comes after it (namely $\displaystyle a_2$). So you use the given formula $\displaystyle a_{k+1}=a_k-4$, substitute the current value ($\displaystyle a_1 =28$ as $\displaystyle a_k$) to find the succeeding value ($\displaystyle a_{k+1} = a_{2}$) and so on...