Practice Object literals-How do I access a global object within a function?

These are my instructions:

Create a function named addInventory(). The function should accept 2 parameters -- the product object, and the number to add to the inventory. The function adjusts the product object's inventory property by adding the number passed into the function. For example, if the inventory of the product is currently 2, calling addInventory(product, 3) will set the value of inventory to 5. Add a console.log() message to the function that includes a message that looks something like this: "3 chairs added to the inventory" In this example, "chair" is the name of the product.

 var product = {
  name: "SkullCandy Headphones",
  inventory: 6,
  unit_price: 49.99
}

addInventory(product, numOfProducts) {
this.inventory + numOfProducts
console.log(numOfProducts + this.name + 'added to inventory');
}

addInventory(product, 6);

What could I be doing wrong? I'm trying to access what seems to be a global variable, and the function is OUTSIDE the object, so I'm not sure if I'm supposed to be using 'this'. It's just my best guess at this point. Is there any valuable info you can provide me to help me out here? Atm it's outputting a bunch of module information when I log it to the console.