First, write out the multiples of 6 and 26. Then find the common multiple they share that is immediately less than 85. (I do 26 first since it will have less multiple and it will give me an idea of what to look for within the multiples for 6)
26: 26, 52, 78 (anything more will be over 85 and we need a number less than 85)
6: 6, 12, 18, 24, 30, 36, 42, 48, 56, 72, 78 (I stopped at 78 since any number after this will be larger than 85)
I now look to find the multiple that both 26 & 6 shares that is also less than 85 and it landed me on the number 78!
The Answer: 78
Hope this helps!