What mass of HF must react to form 51.34 grams of SiF4
I don't know what reaction you're using. One such reaction is as follows:
SiO2 + 4HF ==> SiF4 + 2H2O
moles SiF4 needed = grams/molar mass = 51.34/molar mas SiF4 which you can look up.
Then mols HF = moles SiF4 x (4 moles HF/1 mol SiF4) = mols HF x 4 = ?
Mass HF = moles HF x molar mass HF = ?
Post your work if you get stuck.