GoalSeek Method
Calculates the values necessary to achieve a specific goal. If the goal is an amount returned by a formula, this calculates a value that, when supplied to your formula, causes the formula to return the number you want. Returns True if the goal seek is successful.
expression.GoalSeek(Goal, ChangingCell)
expression Required. An expression that returns a Range object. Must be a single cell.
Goal Required Variant. The value you want returned in this cell.
ChangingCell Required Range. Specifies which cell should be changed to achieve the target value.
Example
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")