x+y=14,y=14-x,(1)
x^3+x^2y-xy^2=392,(2)
把（1）代入（2）,
14x^2-x(14-x)^2=392,
x^3-42x^2+196x+392=0.
x1≈7.1967,或x^2-34.8033x-54.4689≈0,
x2≈36.3037（舍）,x3<0(舍）.
代入（1）,y1=6.8033,
长方形面积xy≈48.961.