The underlying reaction is:
2BCl3+3H2⟶2B+6HCl
Mass of boron formed = 21.6 g,
Moles of boron: massmolar mass=21.6 g10.8 g/mol=2 mol
Two moles of boron are formed, which means that 3 mol of hydrogen gas is consumed from the stoichiometry above.
Since, 1 mol of gas at STP occupies 22.4 L, 3 moles would occupy:
3×22.4=67.2 L