To find the greatest horizontal distance (range) that a cannonball can be shot from a cannon, we can use the formula for the range of a projectile launched at an angle \( \theta \) with an initial velocity \( v_0 \):
\[ R = \frac{v_0^2 \sin(2\theta)}{g} \]
Where:
- \( R \) is the range,
- \( v_0 \) is the initial velocity (350 m/s in this case),
- \( g \) is the acceleration due to gravity (approximately \( 9.81 , \text{m/s}^2 \)),
- \( \theta \) is the launch angle.
To maximize the range, the angle \( \theta \) should be \( 45^\circ \). At this angle, \( \sin(2\theta) = \sin(90^\circ) = 1 \).
Substituting \( \theta = 45^\circ \):
\[ R = \frac{(350)^2 \cdot 1}{9.81} \]
Calculating \( 350^2 \):
\[ 350^2 = 122500 \]
Now substitute that back into the range formula:
\[ R = \frac{122500}{9.81} \approx 12482.24 , \text{m} \]
So, the greatest horizontal distance the cannonball can be shot is approximately \( \boxed{12482.24} , \text{m} \).