just store the direction the enemy is moving at like this
dir = velocity.x/abs(velocity.x)
the abs return the positive value like |velocity.x|
the dir will then be either +1 or -1
and when instancing, remember to store the instance in a temp var so the properties can be changed.
var temp = bullet.instance()
owner.add_child(temp)
now you can access the properties of the bullet and be like
temp.position = pos
temp.velocity.x *= dir
the bullet needs to have something like:
position.x += speed *velocity.x
This is a lazy way to make it work and there are many other ways to do it though.