Beginning with the first part of the equation, you know that you have to subtract 7 because it says "7 fewer than". Any time you see the words "fewer than" in a word problem, that indicates subtraction. For the second half of the equation, you know you have to add because it says "the sum of 6 and s". Any time you see the words "the sum of", it indicates addition. This means that you are adding 6 and s and then subtracting 7. Also, be sure to add in parentheses so that you know what comes first.
So this means that you answer is:
(6 + s) - 7
I hope that helps :)