If z represents the variable of the standard normal distribution curve N(0,1), which is symmetrical about z=0, then we look for P(z)-P(-z)=0.9642=1-2P(-z) => P(-z)=(1-0.9642)/2=0.0179 => -z=-2.099192 => z=2.099192 Check: P(2.099192)-P(-2.099192)=0.9642 ok