what is 8.0 * 10 to the negative 4thpower?

10^-1  =  0.1                (one decimal place)

10^-2  =  0.01               (two decimal places)

10^-3  =  0.001             (three decimal places)

10^-4  =  0.0001            (four decimal places)

So:  8.0 x 10^-4  =  8.0 x 0.0001  =  0.0008