,How can I rotate object on Axis.x same amount as Axis.Z

I am working on AI for a 2D shooter plane game and I am having small issues. I use this code to look at a target on Z axes only, and then rotate on Axis.Z

var newRotation = Quaternion.LookRotation(transform.position - target.position, Vector3.forward);
var temp = newRotation;
	newRotation.x = 0.0;
	newRotation.y = 0.0;
	transform.rotation = Quaternion.Slerp(transform.rotation, newRotation, Time.deltaTime * RotationSpeed);

Here is my question….
How can I rotate object on Axis.x same amount as Axis.Z

newRotation.x = newRotation.z; ???