We can start by looking at what we know.
We know that volume is equal to length x width x height. Filling in this equation with the values we're given:
20cm x width x 30cm =
3600cm^3
If we divide both sides by 20cm we get:
width x 30cm = 180cm^2
Next we can divide both sides by 30cm to get our answer:
width = 6cm