Junes Verne in 1865 suggested sending people to the Moon by firing a space capsule from a 255 m long cannon with a launch speed of 11.42 km/s. What would have been the unrealistically large acceleration experienced by the space travelers during launch?

Use the formula

V = sqrt(2aX)

In this case, solve it for a.
X = 255 m
V = 11.42*10^3 m/s