The Avogadro's number is the constant number for the number of atoms or molecules represented in 1 mol, this number is equal to 6.022*10^23, to find out how many molecules there are in 6.0 moles we can set up this equation:
6.0 moles of CH4 * 6.022*10^23 molecules / 1 mol of CH4
The answer will be 3.61*10^24 molecules of CH4