How do I convert 2.4 x 10^21 formula units of magnesium hydroxide to grams?

Are you taking chemistry at the same time? If not, this is a pretty hard question. 

Facts

1. As a formula unit,  Magnesium Hydroxide has a chemical formula of Mg(OH)2 
2. That means for every Magnesium, there are 2 oxygens and 2 hydrogens
3. The weights of these components are found on a periodic table. Since every periodic table is different, I'm going to approximate the results.

Mg = 24
O  = 16
H = 1
 
4. One mole of anything has 6.02 * 10^23 (in this case) formula units associated with it.

Step One
Find the number of grams in 1 formula unit using the numbers from the periodic table.

1 Mg = 1 * 24 = 24
2 O = 2 * 16 = 32
2 H = 2 * 1 = 2
Total = 58 grams

So 
1 mole of Mg(OH)2 = 58 grams

1 mole / 6.02 * 10 ^23 formula units =  x mole / 2.4 * 10 ^ 21 formula units
Cross multiply

1 mole * 2.4 * 10^21 formula units = x mole * 6.02 * 10^23 formula units.
Divide both sides by 6.02 * 10^23

1 * 2.4 * 10^21 / 6.02 * 10 ^ 23 = x
x = 0.003987 moles

Now do this again for grams
1 mole / 58 grams = 0.003987 mole / x grams  Cross multiply
1 mole * x   = 0.003987 * 58 grams
x = 0.2232 grams <<<<< Answer