Answer:
vcannon = 11.42 [m/s]
Step-by-step explanation:
To solve this problem we must use the principle of conservation of linear momentum which tells us that the momentum of the cannon ball is conserved before and after shooting.
This is at the time of the shot we have two movements, that of the cannon ball moving to the East, while the cannon moves to the left due to the reaction caused by the explosion.

where:
mcannon = mass of the cannon = 700 [kg]
vcannon = velocity of the cannon [m/s]
mcannonball = 20 [kg]
vcannonball = 400 [m/s]
Now replacing:
![0=-(700*v_(cannon))+(20*400)\\v_(cannon)=11.42 [m/s]](https://img.qammunity.org/2022/formulas/physics/college/2arvtuf2o7j68j7gm5287r9jjg66vah4ys.png)