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
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Can u check ur answer once? It is given 4.70 cm in my book?
Volume u got but height?
here volume = height*area
the answer is correct as far as I know...
Thanks anyways