Under the given condition, the answer (the solution) is not unique. For example, X = 1, Y = 3 gives X^3 + Y^3 = 28. X = 2, Y = 2 ====> X^3 + Y^3 = 16. X = 5, Y = -1 ====> X^3 + Y^3 = 124. To have a unique solution (the unique answer), additional restriction/restrictions should be imposed.