The longest straight line that can be drawn to connect two points on the circumference of a circle is the diameter of the circle. The diameter can be calculated using the radius.
The formula for the diameter \( d \) of a circle is:
\[ d = 2 \times r \]
Where \( r \) is the radius of the circle. Given that the radius \( r \) is 9 inches, the diameter would be:
\[ d = 2 \times 9 = 18 \text{ inches} \]
Therefore, the longest straight line that can be drawn to connect two points on the circumference of the circle is 18 inches.