$\displaystyle

\frac {\sigma_z}{z} = \frac {\sigma_x}{x} + \frac {\sigma_y}{y}

$

For z=x+y

where x=30cm +/- 5cm

and y=40cm +/- 8cm

I don't understand what to do here, I guess I don't know what they are asking. How does this get set up?

How do I do the z, x, and y parts? Do I need to do it 2x, once for + and another for -?