All you have to do here is use Avogadro's number to convert the number of moles given to you to number of molecules.
Notice that the problem provides you with the molar mass of the amino acid. You don't need it to figure out how many molecules you have in your sample.
By definition, one mole of a molecular compound contains
#color(blue)(|bar(ul(color(white)(a/a)"1 mole" = 6.022 * 10^(23)"molecules"color(white)(a/a)|))) ->#Avogadro's number
In your case, the sample is said to contain
#3 * 10^(-21) color(red)(cancel(color(black)("moles"))) * (6.022 * 10^(23)"molecules")/(1color(red)(cancel(color(black)("mole")))) = color(green)(|bar(ul(color(white)(a/a)color(black)("1800 molecules")color(white)(a/a)|)))#
You should round this off to one sig fig, since that's how many sig figs you have for the number of moles, but the listed answer is rounded to two sig figs, so I'll leave it like this.