thanks !

You have the scale bar at the top.

That entire length has a value of 1um (You measure this in real life with a ruler. I assume it's length is 45 mm)
So for every 45mm in real life, represents 1um in the picture.

You measure the length of X--> Y which from mark scheme 18 mm (But measure this with a ruler in real life)

Next you work out how X-->Y compares to the value of the scale bar.

So if 45 mm = 1um
X-->Y (18mm) = 0.4um

(X-->Y) / (Scalebar) = thickness
(18) /(45) = 0.4um