I have come across this question too and find it a little confusing..
The book I'm studying from says:
-Find pressure altitude
-Then find indicated height AGL
-Set pressure altitude against airport temp.
-on the outer scale of the whizz wheel, read true height AGL, opposite indicated AGL
-If necessary, add airport elevation to true height AGL to get true height AMSL.
So with this question I would have thought you would set -14 against the pressure altitude of 6,780ft and read off a figure opposite 580ft on the outer scale.
But obviously that is wrong as being approx ISA-15, one would expect the altimeter to over-read by more than the answer of 560ft AGL this method yields.