Unable to change value of "Spring Length" in code

Godot Version

4.6

Question

i am trying to change the value “Spring Length” in the node “Spring Arm” but it says

Identifier “spring_length” not declared in the current scope. why and how do i fix it

extends Node3D

const MOUSE_SPEED = -0.001
const JOY_SPEED = -3

const MAX_ZOOM = 1
const MIN_ZOOM = 100

func _ready() -> void:
	#Input.set_mouse_mode(Input.MOUSE_MODE_CAPTURED)
	pass

func _process(delta: float) -> void:
	#var joy_dir = Input.get_vector("cam_left", "cam_right", "cam_up", "cam_down")
	#rotate_from_vector(joy_dir * JOY_SPEED * delta)
	
	if Input.is_action_just_pressed("scroll_down"):
		spring_length += 1                            #this is giving an error

	
	Global.camera_dir = rotation
	
	if Input.is_action_just_pressed("click"):
		Input.set_mouse_mode(Input.MOUSE_MODE_CAPTURED)
	if Input.is_action_just_pressed("exit"):
		Input.set_mouse_mode(Input.MOUSE_MODE_VISIBLE)

func _input(event: InputEvent) -> void:
	if event is InputEventMouseMotion:
		rotate_from_vector(event.relative * MOUSE_SPEED)
		
func rotate_from_vector(v: Vector2):
	if v.length() == 0: return
	rotation.y += v.x
	rotation.x += v.y
	rotation.x = clamp(rotation.x, -1.5, 1.0)
	#print(rotation_degrees.x)

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i figured out what i did wrong i should have it say extends SpringArm3D not Node3D