If I don't know the mean and the standard deviation how can I find the area of z-score -0.75

Answer 1

I thought I had given you the answer.

finding the z-score results in a mean of 0 and a standard deviation of 1
so in the link I gave you ....
http://davidmlane.com/hyperstat/z_table.html

enter 0 for the mean
1 for the sd
click on "below" and enter -.75

click on "calculate" to get .2266