The numbers are small enough to actually count them.

There are 36 possible combinations (6×6)

For the sum to be 2, there is only 1 combination: 1-1

For the sum to be 3, there are 2 combinations: 1-2 and 2-1

For 4, there are 3 combinations: 1-3, 2-2, and 3-1

For 5, there are 4 combinations: 1-4, 2-3, 3-2, and 4-1

For 6, there are 5 combinations: 1-5, 2-4, 3-3, 4-2, and 5-1

So, for the sum to equal 6, the probability is 5/36

For the sum to be at most 6, the probability is (1+2+3+4+5)/36 = 15/36

I will leave the last one for you!