First we will convert all the units in terms on centimetre

actual height of chimney = 12m = 12*100= 1200 cm

actual height of the factory= 28m = 28*100= 2800cm

ratio of height of model chimney to height of actual chimney = 9/1200

= 3/400

height of model factory= ratio of height of model chimney to height of actual chimney* height of actual factory

=3/400*2800

=21cm or 0.21m