How many moles are in 2.16*1024 atoms of magnesium?

1 mol of a substance is made of 6.022 x 10²³ units, these units could be atoms making up an element or molecules making up a compound.

Mg is an element therefore 1 mol of Mg is made of 6.022 x 10²³ atoms of Mg.

so if 6.022 x 10²³ atoms make up - 1 mol of Mg

then 2.16 x 10²⁴ atoms make up - 1 / (6.022 x 10²³) x 2.16 x 10²⁴ = 3.59 mol

therefore there are 3.59 mol of Mg