To find the equation of the graph representing the relationship between weight and cost, we can use the formula for direct proportionality:
\[ C = k \cdot W \]
where \( C \) is the cost, \( W \) is the weight, and \( k \) is the constant of proportionality.
From the graph, we can see that the line passes through the point \( (2, 10) \). This means that when the weight \( W = 2 \) ounces, the cost \( C = 10 \) dollars.
Now we can substitute the values of \( C \) and \( W \) into the equation to solve for \( k \):
\[ 10 = k \cdot 2 \]
To find \( k \), we divide both sides by 2:
\[ k = \frac{10}{2} = 5 \]
Now that we have the constant of proportionality \( k \), we can write the complete equation as:
\[ C = 5W \]
Thus, the equation of the graph with its constant of proportionality is:
\[ C = 5W \]