After grading a lot. a builder places lour stakes to designate the corners of the slab for...

After grading a lot. a builder places lour stakes to designate the corners of the slab for a house. To provide a  frim, level base for the slab, the builder places a minimum of 3 in. of gravel beneath the slab. Determine the volume of gravel needed and the xcoordinate of the centroid of the volume of the gravel. (Hint:The bottom surface of the gravel is an oblique plane, which tan be represented by the equation y = a + bx + cz.)