To activate the keyboard backlight on your Asus VivoBook X515JAB, follow these steps:
Locate the "Fn" key on your keyboard. It is usually located in the bottom left corner of the keyboard.
Look for the key that has the backlight icon on it. On the Asus VivoBook X515JAB, this key is typically located on the top row of keys, and it looks like a light bulb.
Press and hold the "Fn" key, and then press the backlight key to toggle the backlight on or off.
Note that some Asus laptops have different backlight settings that can be adjusted using the "Fn" key and the arrow keys. If the above steps don't work for your specific model, you may need to consult your user manual or contact Asus support for further assistance.