I believe the answer is $45.75.
I hope this helps.
If you would like an explanation of how I got this, please reply.
Answer:
The 95% confidence interval for the population mean is between 61.5 and 68.5.
Step-by-step explanation:
We have that to find our
level, that is the subtraction of 1 by the confidence interval divided by 2. So:

Now, we have to find z in the Ztable as such z has a pvalue of
.
So it is z with a pvalue of
, so 
Now, find M as such

In which
is the standard deviation of the population and n is the size of the sample.

The lower end of the interval is the sample mean subtracted by M. So it is 65 - 3.5 = 61.5
The upper end of the interval is the sample mean added to M. So it is 65 + 3.5 = 68.5
The 95% confidence interval for the population mean is between 61.5 and 68.5.
Subtract 2.1 from both sides:

Add 1.4y to both sides:

Of course, we can rewrite this as

Divide both sides by 1.4
