Hey There!
Ideal gas equation: PV = nRT where P is pressure, V is volume, n is moles of gas, R is molar gas constant and T is temperature.
Since P, R and T are constant, V/n = constant
V1 = 2.00 L, V2 = 3.30 L
n1 = (2.00 g)/(4.00 g/mol) = 0.5 mol, n2 = ?
V1/n1 = V2/n2
2.00/0.5 = 3.30/n2
n2 = 0.825 mol
Moles of He added = n2 - n1
= 0.825 - 0.5 = 0.325 mol
Mass of He added = moles x atomic mass
= 0.325 * 4.00 = 1.30 g of He