Public Function F(x)
If x <>  0  Then F = (x ^  3  +  4 ) / x ^  2 
End Function
Sub mysub()
Dim x As Integer, y As Double, x1 As Integer, h As Double, x2 As Double, z As Double, extremum As Double
x1 = InputBox("Введите значение х1", "ввод х1",  1 )
x2 = InputBox("Введите значение х2", "ввод х1",  3 )
h = InputBox("Введите значение h", "ввод х1",  0 . 2 )

y = x1
extremum =  0 

Do While y < x2
    If F(y) < extremum Then extremum = F(y)
    y = y + h
Loop
Debug.Print extremum
End Sub