the plane is symmetrical about y axis therefore
the depth of the centroid from free surface hc=1.75sin45
=1.24ft therefore 1ft=0.3084m :1.24*0.3084=0.38m
=0.6m at free surface
b)depth below the water surface
depth of the centroid below =45+0.38=45.38
now we can say it's from the water surface=0.1413 is the magnitude from the upper edge of the plate.