Number of mg(oh)2 formula units in 7.40 moles of mg(oh)2.

The answer is 4.45 × 10²⁴ units.

To calculate this, we will use Avogadro's number which is the number of units (atoms, molecules) in 1 mole of substance:
6.02 × 10²³ units per 1 mole

So, we need a proportion:
If 6.02 × 10²³ units are in 1 mole, how many units will be in 7.40 moles:
6.02 × 10²³ units : 1 mole = x : 7.40 moles

After crossing the products:
1 mole * x =  7.40 moles * 6.02 × 10²³ units
x = 7.40 * 6.02 × 10²³ units
x = 44.5 × 10²³ units = 4.45× 10²⁴ unit

A.(nh4)(2co3)is the correct answer