To find the total amount \( a \) that Mike paid for buying 8 yards of red ribbon at a price of $3.50 per yard, we can set up the following equation:
\[ a = \text{(number of yards)} \times \text{(cost per yard)} \]
Given that Mike bought 8 yards, and the cost per yard is $3.50, we can substitute the values into the equation:
\[ a = 8 \times 3.50 \]
Now we can calculate the total amount:
\[ a = 8 \times 3.50 = 28.00 \]
Thus, the complete equation is:
\[ a = 8 \times 3.50 \]
And the total amount \( a \) that Mike paid is:
\[ a = 28 \text{ dollars} \]