well...i did it this way.....
the temp gradient will be...
T/x 
+ T/y 
+ T/z 
=2x +2y -
now this is the direction in which max increase in the temp takes place.......
hence fr the fly to cool faster it shud move in the opp direction ie in the direction of -2x -2y +......
sub the pt n ull get the ans as 'b'
