@Ronaldo_ You may want to consider some other factors. One is the number of troops you are bringing.
Theoretically, you can move your siege units (the bombards) into place, fire each one, and then try to take the city with your knight. I'm skeptical that the city would be weakened enough so that you could take it in a one-turn, rapid attack. With only 4 units total, you're definitely vulnerable to an AI or human counter-attack.
A key aspect of city combat is putting the city under siege. That is, placing enough troops on 5 or 6 of the tiles surrounding the city so that the little red icon appears in the city name. The impact is that neither the city health, nor the wall health, will recover/heal while under siege. In my single-player experience, nearly every time I attack a city with walls, I need to surround it, get it under siege, and spend at least 2 turns pounding on the walls. If I've brought a battering ram or siege tower, then my normal attack units can do more damage to the city itself.
Since you're playing vanilla (not Rise & Fall), your target city won't have the Victor governor promotion to keep it from being put under siege. But bringing more units, controlling more tiles, and being able to absorb some punishment from the city for a multi-turn campaign will increase your chances.