There are 36 possible combinations of rolling 2 dice (6*6)

There are 5 possible ways of throwing a 6, (1,5),(2,4),(3,3),(4,2),(5,1)

There are 6 possible ways of throwing a 7, (1,6),(2,5),(3,4),(4,3),(5,2),(6,1)

So there are 11 possible ways of rolling a six or seven out of 36 possible rolls.

The probability is 11/36.