Answer:
The excluded value is c=0
Step-by-step explanation:
The excluded value is when the denominator goes to zero
(6d^(2)c)/(3c)
3c = 0
c =0
The excluded value is c=0
Looks like you just evaluated the summand for the given value of

, whereas the question is asking you to find the value of the sum for the first

terms.
Let

. Then

is the

th partial sum.

happens to be the first term in the series, which is why that box is marked correct:

But the next partial sum is not correct:

and this is not the same notion as the second term (which indeed is 0.75) in the series.
75x + 90(15-x)=88(15)
75x+1350-90x = 1320
-15x + 1350 = 1320
-15x= -30
x=2
the amount of 75 octane is 2 gallons
the amount of 90 octane is 13 gallons
Hope this helps.