At this rate, what elevation will kelli reach 7 hours after starting?

Here, your A.P. is 7000, 7600
Now, You need to calculate the number at 7th place, 
We have: a = 7600, d = 7600 - 7000 = 600, n = 7
a(n) = a + (n-1)d
a(7) = 7600 + (7-1) 600
a(7) = 7600 + 3600
a(7) = 11200 feet

In short, Your Answer would be Option C

Hope this helps!

10feet apart then both

That. you would have multiple