I want to purchase the following memory upgrade:
how can i know if it will fit with my dell precision 5510 - intel core i7-6820HQ ?
You'll need to ask Corsair (or your vendor) about compatibility.
You do not need memory at 3 GHz -- if it does work, it'll simply downclock to your system's rated speed. The added speed is designed for overclocking (which your system can't do).