¡Need help with these Henderson-Hasselbatch equation problems!?

Don't need you to answer tue problems, just give me a point of where to start, i know i have to apply the Henderson-Hasselbatch equation, but i don't know where to start, any help will be thankfull.

1. To maintain a stable hydroponics is preferable to provide a buffer for irrigation rather than pure water. This is because of the osmotic pressure generated by the presence of salts that allows migration of water into the plant cells. So, you need to prepare a buffer solution of 0.5M phosphates, pH 7. You have NaH2PO4 and Na2HPO4. How many kg of each salt you need to prepare 100L of this buffer?

2. Buffering is essential for growing animal tissues in lab. An abrupt change in pH damages cells causing their death. For an experiment, you need to prepare a phosphate buffer, however, you have not any salt. Instead, you have a 5M Na2HPO4 and 4N HCl solutions. Which volume of each solution you need to make 1L of buffer 0.3M, pH 7.2?

3. To extract a protein from yogurt you need a buffer of pH 4. You have 4M acetic acid solution and solid NaOH. How many ml of acetic acid and how many grams of NaOH do you need to prepare 500ml of 0.3M buffer?