Well, you already are! I can see that return 180; inside class Car's method test()!
Problem is that you need something to "capture" the returned value after calling it!

Within function draw() block, you call Car's test() using myCar as reference of an instance of Car.
However, even though the "action" inside that happens normally -> background(0);
there's nothing to receive its returned value!

After that, you try to display the contents of a non-existent variable test!
The program bugs displaying -> "Cannot find anything named test"

Fix suggestion:
Use println() to display the returned value of method test(), like this -> println( myCar.test() );
Or, if you intend to use that later, store it in a variable 1st, only then display it!

Laterz...  

To supplement GoToLoop's answer here is a more realistic Car class demonstrating getters and setters.

1. Car myCar;
2. int currSpeed;
3. 
   
4. void setup() {
5.   size(300, 300);
6.   myCar = new Car();
7.   myCar.setSpeed(50);
8. }
9. 
   
10. void draw() { 
11.   background(0);
12.   fill(0,255,0);
13.   myCar.setSpeed(mouseX / 3);
14.   currSpeed = myCar.getSpeed();
15.   text(currSpeed, 20,30);
16. }
17. class Car {
18.   private int speed;
19.  
20.   public void setSpeed(int s){
21.     speed = s;
22.   }
23.  
24.   public int getSpeed(){
25.     return speed;
26.   }
27. }

many thanks for your precious help