You choose which way you want to calculate it. You can take the 17 +2 from+ 2 (from Tortle AC + shield) or you can pick 13 + DEX + 2 from sorcerer(from Draconic Sorcerer AC + shield).
The basic rules say
Some spells and class features give you a different way to calculate your AC. If you have multiple features that give you different ways to calculate your AC, you choose which one to use.
This means it's your choice which to pick.