To set up the working equation for the amount \( a \) that Mike paid for buying 8 yards of ribbon, we start by recognizing that the cost per yard is $3.50 and Mike bought 8 yards.
The total cost \( a \) can be represented by the equation:
\[ a = \text{(cost per yard)} \times \text{(number of yards)} \]
Substituting the values:
\[ a = 3.50 \times 8 \]
Now, let's calculate the total amount:
\[ a = 3.50 \times 8 = 28 \]
So, the final answer is:
\[ a = 28 \text{ dollars} \]