When rolling two six faced dice, what is the probability the sum equal to seven?

What you want is P(6∩1) or P(1∩6) or P(2∩5) or P(5∩2) or P(3∩4) or P(4∩3).
The events of rolling the dice are independent (i.e. they don't affect one another) so:
E.g.
P(6∩1) = P(6) * P(1)
P(2∩5) = P(2) *  P(5)
The probability of getting a given number on a roll is 1/6 for both dice.
So:
P(6∩1) = 1/6 * 1/6 = 1/36
This is the same for any arrangement of numbers you could get from rolling two dice.
So, we can see that there are 6 arrangements of numbers that will give a sum of 7 and so that is 6 * 1/36 = 6/36 = 1/6