I am having a hard time starting my solution to the following problem:

So far I have been looking throughout the text book at some sample problems but the sample problems are not asking for a similar solution to this question. I have also referred to my lecture notes and have not gotten any where for quite some time. Mentally I thought that I could find the volume of the silicon oxide but I was incorrect in my mental calculations. Please help me get started.

From the density and thickness, you can get the mass in a square millimeter of oxide. From there, you can get the number of Si and O atoms by knowing how much these atoms weigh.