Slope is an average. In this particular context, the slope of 6.9 (or 7) means that an average of 7 people entered the store every hour.
If x is the number of hours the store is open and we are told to find the number of customers that have entered the store after 5 hours, we fill in the equation as follows:
y = 7(5) + 10 and
y = 35 + 10 so
y = 45.
If you use 6.9 instead of 7:
y = 6.9(5) + 10 and
y = 34.5 + 10 so
y = 45.5
Since you can't count a half of a person, using 7 instead of 6.9 makes more sense.
