x^3 – (7 + x)^3 > -21x2 – 490.

x^3 - 49 * 7 - 21 x^2 - 3*49*x - x^3 >  -21x2 – 490

49 * 10 - 49 * 7 >3 * 49 * x

x * 49 * 3 < 49(10 - 7)

x * 49 * 3 < 49 * 3

x < 1