Here's what I did which might help you
Slope intercept form
y = mx + b
-2x + 5y = 10
5y = 2x + 10
Divide by 5
y = 2/5x + 2
The empirical rule says that approximately 68% of a normal distribution falls within one standard deviation of the mean. In this case, this would include all values of the variable in the interval

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8208 total tickets / 12 concerts =
684 tickets per concert.