This problem is killing me.
Determine the number of atoms thick in your sample of Al by using the thickness of your sample and the thickness of one atom. One Al atom is 2.5*10^-8cm thick. (In other words, how many atoms are stacked upon each other to make up the thickness of your sample?)
Please help me.

What is the thickness of your Aluminium sample?

Use a micrometer screwgauge to get that thickness.

Once you got it, use proportions.
2.5*10^-8 cm -> 1 atom
1 cm -> 1/(2.5*10^-8) atoms
? Cm ->? Atoms