To change units, try drawing a diagram. Suppose you want to change
\[50cm^2\]
into
\[m^2\]
, which the usual unit. May people would divide by 100 because there are 100 centimetres in a metre.
THIS IS WRONG!
Draw a recjangle with sides labelled in cm that multiply to give 50.
Change each side into metres, then multiply to give the area in