If I understand the question correctly, you want to know the z that corresponds to an x value of 8 that is 1.6 standard deviations from the mean.
That means that the mean*1.6 = 8, which means that the mean is 8/1.6 = 5
Z = (x-mean)/standard deviation = (8-5)/1.6 = 1.875
Please let me know if I misunderstood