If the average temperature in October is 59.3 degrees with a standard deviation of 2.6 degrees, what is the probability that the temperature will be over 75 degrees?

Use z=\frac{x-{\mu}}{\sigma}

Look up the probability corresponding to the z score found above, then subtract from 1.

I did that, but on my z score chart, it only goes up to 3.9 and I got an annswer with a 6 for the z score

This only means that it is highly improbable, near impossible :)