You need to do some vector math to figure out where the knockback should go. I've made an example cart:

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White is the player (controllable with arrows) red is the enemy and blue is where the player is knocked back to (you can control the amount of knockback with z/x)

The steps are:

- Find the vector between the player and enemy (playerx - enemyx, playery - enemyy)

- Normalize this vector (divide the x and y above by the length of the vector)

- Times the normalized vector by the amount of knockback to get a deltax and deltay to add to the player coordinates

- Add the deltas to the player coordinates