Original Title: Dell Inspiron 3421 5421 Laptop Motherboard Mainboard PTNPF Optimized Title: Dell Inspiron 3421 5421 Replacement Laptop Motherboard - PTNPF Mainboard for Repair & Upgrade | Compatible with Inspiron 14 3000 Series (Note: The original Chinese title was already in English, so no translation was needed. I added keywords like "Replacement," "Repair & Upgrade," and the compatible model series for better SEO. The use case highlights repair and upgrade scenarios.)