mass 0.49g density 2.70g/cm^3 size of alumminum atom is 2.5*10^8cm length=12 cm
width =12cm

In order to answer that question, you need to answer three simpler questions. If you get stuck on one of them, let us know. Somebody will give you further guidance.

1) If 1 cm^3 of aluminum weighs 2.7g, how many cm^3 do you have in 0.49g?

2) Once you know the volume of aluminum you have, how thick does a 12cm x 12cm sheet have to be in order to comprise that much volume?

3) Once you know the thickness in cm, how many atoms does at take to span that distance?