the link doesn't seem to be working for me but taking hints from the conversation already going on, i presume that the events are made such that the zombie will always go towards the right
what needs to be done here is to create additional conditional checks
+ is for conditions
+ zombie collides with wall
+ zombie is facing towards the right
- set the direction to face left (or bounce towards the left if you can)
+ zombie collides with wall
+ zombie is facing towards the left
- set the direction to face right (or bounce ...)
a little advice for future problems:
whenever you are stuck with a problem, try to break down as much as possible or to the point that you can understand the problem well enough to tackle the smaller pieces that make up the entire problem