Let height of the block immersed be 'x'
height of block above surface will be 10-x
apply archimedes' principle
buoyant force acting on body = weight of body
density of mercury * volume of block immersed * g = density of block * volume of block* g
13.6*10*10*x = 7.2*10*10*(10-x)
simplifying you get x=3.67
there fore height of block above mercury surface = 10-x = 10-3.67 = 6.33