One mole any substance contains 6.022 ₓ 10²³ particles called Avogadro's Number.
The relation between moles and number of particles is given as,
# of particles = moles ₓ Avogadro's number
In our case the particles are formula units of MgCO₃. So, 1 mole of MgCO₃ contain 6.022 ₓ 10²³ formula units, then the number of formula units in 1.72 moles are calculated as,
# of formula units = 1.72 mol ₓ 6.022 ₓ 10²³ formula units / mol
# of formula units = 1.035 ₓ 10²⁴ Formula Units