Um.. Both of those equations are the exact same. Divide both sides of the first equation by 6. You get:

That is the exact same as the second equation. This system has an infinite number of solutions. 5x - 7y = -1 is a line, so basically every point on that line is a solution to the system.
For example,

and

would work, but so would

and
So to find the mean you add up all the numbers (or the exam scores) which will get you 212 then you take the number and divide it by the total numbers in the set (how many numbers did you add up?)
212 divided by 8 is 26.5 which would be C. I hope this helps!
The missing exponent is 21 :)
Explanation: when dividing exponents, subtract them so in this case, it’s 25-19 to get 6 as the answer