`a)` `5x^2 y-10xy+15xy^2`
`=5xy. x-5xy . 2 +5xy. 3y`
`=5xy(x-2+3y)`
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`b)` `6x^2 y^4-15x^3 y^2+9x^3 y^3`
`=3x^2 y^2 . y^2-3x^2 y^2 . 5x+3x^2 y^2 .3xy`
`=3x^2 y^2 (y^2-5x+3xy)`
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`c)` `18x^4 y-12x^3 y^2+24x^2`
`=6x^2 . 3x^2y -6x^2 . 2xy^2+6x^2 .4`
`=6x^2 (3x^2 y-2xy^2+4)`