Reqd. area = (x^{2 }– 4xy + 7y^{2}) (x^{3} – 5xy^{2})
= x^{2}(x^{3} – 5xy^{2}) – 4xy (x^{3} – 5xy^{2}) + 7y^{2 }(x^{3} – 5xy^{2})
= x^{5} – 5x^{3}y^{2} – 4x^{4}y + 20x^{2}y^{3} + 7x^{3}y^{2 }– 35xy^{4}
= x^{5 }+ (7 – 5)x^{3}y^{2} – 4x^{4}y + 20x^{2}y^{3} – 35xy^{4}
= x^{5} + 2x^{3}y^{2} – 4x^{4}y + 20x^{2}y^{3} – 35xy^{4}
= (x^{5 }– 4x^{4}y + 2x^{3}y^{2 }+ 20x^{2}y^{3} – 35xy^{4}) sq. unit.