vb 这有什么问题？

不是spr是sqr,开方!!

Private Sub Command1_Click()
Dim a As Single
Dim b As Single
Dim c As Single
Dim d As Single
Dim x1
Dim x2
a = Text1.Text
b = Text2.Text
c = Text3.Text
d = b * b - 4 * a * c
If d >= 0 Then
x1 = (-b + Sqr(d)) / (2 * a)
x2 = (-b - Sqr(d)) / (2 * a)
Text4.Text = x1
Text5.Text = x2
Else
x1 = "无实数解"
x2 = "无实数解"
Text4.Text = x1
Text5.Text = x2
End If

End Sub

你这样的声明：
Dim a, b, c, d, e, x1, x2 As Single

只有 x2 是 Single 类型，a, b, c, d, e, x1 都是变体类型。

建议改为：
Dim a As Single , b As Single , c As Single , d As Single , e As Single
Dim x1, x2