You can approach this quest only after finishing the first quest for Esbern, namely (The Blades) Ask Esbern about dragon lairs. Esbern will ask you to bring him one Dragon Scale and one Dragon Bone. If you followed my advice and examined the body of the recently killed dragon (screen above), you should already have them. Otherwise you can return there, or anywhere else you fought a dragon before (or eventually kill a new one). Give Esbern the ingredients and in return he will give you Esbern's Potion. Drink it to lower the damage dealt by dragons by 10%.