I need help with this exercise i don't understand good

```
function Circle (radius) {
this.radius = radius;
this.area = function () {
return Math.PI * this.radius * this.radius;
};
// define a perimeter method here
function perimeter(radius){
return 2*(Math.PI)*this.radius
}
};
```

Define a method perimeter that calculates the perimeter of a circle.

help me please