how to do 14.2 Q4 
please tell 

as ab =3.9cm.
so, first draw a line of 3.9 am and name the ends as a and b.
next, you draw a line about 8 cm.
now , take a compass and set the distance of 3.9 cm and cut once on the line drawn of 8 cm
then, you have cut again from the point where you had cut previously( 3.9 cm ) 
now name it as line PQ.[ P IS ON ONE END AND Q IS ON THE SECOND CUT{3.9*2=1.8 CM}]

HOPE THIS HELPS YOU !!!!