I have the following:

• x is mean of X={4,6} and equals to 5 and standard deviation of X equals to 1.41
• y is mean of Y={3,9} and equals to 6 and standard deviation of Y equals to 4.24

How can I calculate the standard deviation of z which is equal to:

$$ z=\frac{y-x}{x}100 = \frac{6-5}{5}100=20$$