To find the amount \( a \) that Mike paid for 8 yards of red ribbon, we can set up the equation based on the cost per yard and the total yards bought.
The cost per yard is $3.50, and Mike bought 8 yards. Thus, the equation can be written as:
\[ a = 8 \times 3.50 \]
Now we can calculate \( a \):
\[ a = 8 \times 3.50 = 28.00 \]
Therefore:
Working equation: \[ a = 8 \times 3.50 \]
Amount paid: \[ a = 28 \text{ dollars} \]