How do I find $\displaystyle P(Z \le -0.9)$? I think it is the same as $\displaystyle P(Z \ge 0.9)$. So then, do I just find $\displaystyle \Phi$0.9 and subtract it from 1? (1-0.8159=0.1841)
I really like these online stat calculators.
They give p-values and they have all kinds of densities, t, F...
Free Cumulative Area Under the Standard Normal Curve Calculator
Plugging in -.9 give a probability of .1840601 and you can see a pretty picture too.