To determine the total amount \( a \) that Mike paid for 8 yards of ribbon, we can set up the working equation based on the cost per yard and the number of yards purchased.
- The cost per yard of ribbon is $3.50.
- Mike bought 8 yards.
The total cost can be represented by the equation:
\[ a = 8 \times 3.50 \]
Now, let's calculate \( a \):
\[ a = 8 \times 3.50 = 28 \]
So we can complete the equation as follows:
\[ \frac{a}{8} = 3.50 \]
Finally, we have:
\[ a = 28 \text{ dollars} \]