Answer:
T = 100 C (212 F)
Step-by-step explanation:
'Water of hydration' is the terminology used to refer to the water molecules present in the crystals of a given substance that can be removed by heating the substance without altering its composition.
MgNH4PO4.6H2O is essentially magnesium ammonium phosphate which has 6 molecules of water of hydration. These can be removed stepwise by gradually increasing the temperature. At a final temperature of 100 C or 212 F the compound will lose all water of hydration.