(x-7)^3+42x^2=(x+7)^3+14-7x
x^3-21x^2+147x-343+42x^2=x^3+21x^2+147x+343+14-7x
-21x^2-343+42x^2=21x^2+343+14-7x
21x^2-343=21x^2+357-7x
-343=357-7x
7x=357+343
7x=700
x=100
(x-7)^3+42x^2=(x+7)^3+14-7x
x^3-21x^2+147x-343+42x^2=x^3+21x^2+147x+343+14-7x
-21x^2-343+42x^2=21x^2+343+14-7x
21x^2-343=21x^2+357-7x
-343=357-7x
7x=357+343
7x=700
x=100