I can do everything else but chem (:

Let's see if I can walk you through these. They're actually not that hard. I'll do the first one for you, but I won't do the rest.

Okay: you know that this is a double-replacement reaction. Double replacement reactions go as follows:

xAB+xCD --> xAD+xCB

So what you have to do to figure out double-replacements is break the compounds down to their original ions, then swap the cation in the first compund with the cation in the second compound, make sure that the new compounds created have the correct ratio, and balance your equation. For example, take your first problem. SiO2 is seperated in to the cation Si (2+) and the anion O2 (2-) (this is peroxide, a different ion than plain oxygen); HF seperates into the cation H (1+) and the anion F (1-). Since the cations swap, your new compounds are SiF2 and H2O2. Don't forget to make sure the ratios of your new compounds work out: H has a charge of positive one, while O2 has a negative two charge, so there need to be two H to every O2 in the compound. The same principle applies to the silicon compound. in the end the new equation ends up being:
SiO2 + 2HF --> H2O2 + SiF2

Notice that I balanced the equation. About the ammonia equation- I think you're supposed to assume that it is in an aqueous solution, so you need to write out what it does in water (here's a hint: NH3 is unstable and tries to reach the more stable NH4+ form.) Hope this helped!

Aww, your welcome. I'm glad that I could help!