To find 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.
- Let \( a \) be the total amount Mike paid.
- He bought 8 yards of ribbon, and each yard costs $3.50.
The equation representing the total amount he paid can be written as:
\[ a = 8 \times 3.50 \]
Now we can solve for \( a \):
\[ a = 8 \times 3.50 = 28 \]
So, the complete working equation is:
\[ a = 8 \times 3.50 \] and the amount Mike paid is:
\[ a = 28 \text{ dollars} \]