Hi
One way:

=IF(I6>20,I6-20,"")
this will leave the cell blank if I6 is under 20, you could also put text inside the double quotes if you wish... i.e. =IF(I6>20,I6-20,"Ok")

or
=IF(I6>20,I6-20,0)
to display a zero if I6 is under 20

Hope that helps!
JG