what mass of mg must you use to make 5.6x10 to the fourth power of mg(OH)2

It is not clear to me what are the units of your 5.6E4 is, grams, kg, moles, tons?

at any rate, find the percentage of Mg in Mg(OH)2.

percentMg= atomicmassMg/(atomicmassMg+2 atomic mass of O + 2 atomic mass H)

then, massMg needed= massMg(OH)2/precentMg