6 3/6 is "six and three-sixths", or "6 ones plus 3 sixths". You want to get this as a number of sixths. Since a one is 6 sixths, the 6 ones are 6 times 6 sixths. So you have 6 times 6 sixths plus 3 sixths, that is, 36 sixths plus 3 sixths, or 39 sixths, which is just 39/6.