How did they get the (6 / 2) = 15 ways? Same for (8 / 4) = 70 ways? I get that they multiply 15 and 70 at the end, but how would you find the number of ways?

How did they get the 6 2 15 ways Same for 8 4 70 ways I get that they multiply 15 and 70 at the end but how would you find the number of ways class=