help me plz

For a rate you need two units.

like

miles/ hour

or

words/page

You only have one unit ( cm ) so you cannot turn it inato a rate.      

I think you may mean :  15 cm to 12 cm rate of decrease

So we have

[15 - 12 ] / 15  =   3/15 =  1/5   = .20 ......  or 20%  rate of decrease