The idea here is that you need to use the of water at
So, density is defined as mass per unit of volume. In this case, a density of
This means that
$2.46color(red)(cancel(color(black)("mL"))) * "1.00 g"/(1color(red)(cancel(color(black)("mL")))) = "2.46 g"$
Now, water has molar mass of
$2.46color(red)(cancel(color(black)("g"))) * "1 mole water"/(18.015color(red)(cancel(color(black)("g")))) = "0.1366 moles water"$
Finally, the relationship between number of moles and number of molecules is given by Avogadro's number, which tells you that every mole of a substance contains exactly
In this case, you will have
$0.1366color(red)(cancel(color(black)("moles"))) * (6.022 * 10^(23)"molecules")/(1color(red)(cancel(color(black)("mole")))) = color(green)(8.23 * 10^(22)"molecules")$
So,