Going to a smaller unit → multiply. Going to a bigger unit → divide.
Convert 5 cm² into mm²
Hit "show me the steps" if you get stuck.
Step 1 — Which way am I going?…
Step 2 — Use the area factor…
Step 3 — The answer…
mm²
Part 2 · Choosing a sensible unit
Which unit would you measure a school desk top in?
The trick is the area factor, not the length factor. 1 cm = 10 mm, but 1 cm² = 10 × 10 = 100 mm². Pick the unit that gives a tidy, sensible number for the thing you're measuring.