A micrograph of a plant leaf section shows stomata at a magnification of ×400\times 400×400. The measured length of one of the guard cells in this image is 7.2 mm7.2\text{ mm}7.2 mm. Calculate the actual length of this guard cell in micrometres (μm\mu\text{m}μm). Note: 1 mm=1000 μm1\text{ mm} = 1000\ \mu\text{m}1 mm=1000 μm.