A cylinder with a movable piston contains 2.00 of helium,He, at room temperature. More helium was added to the cylinder and the volume was adjusted so that the gas pressure remained the same. How many grams of helium were added to the cylinder if the volume was changed from 2.00 L to 3.10 l ?

Typo in the first line, I suppose you mean 2 grams of Helium
P1 V1 = n1 R T1
P1 V2 = n2 R T1
so
V1/V2 = n1/n2
2/3.1 = 2/x
x = 3.1
x-2 = amount added = 1.1