This example assumes that Sheet1 has a cell named "Polynomial" that contains the formula =(X^3)+(3*X^2)+6 and another cell named "X" that’s empty. The example finds a value for X so that Polynomial contains the value 15.
Worksheets("Sheet1").Range("Polynomial").GoalSeek _
Goal:=15, _
ChangingCell:=Worksheets("Sheet1").Range("X")