VB新手求求各位大神解答，万分感谢！！！

Question

Dim num11 As String
Dim num8 As String
Dim num7 As String
Dim num9 As String
Dim num11 As String
Dim num10 As String
num9 = TextBox11.Text
num10 = TextBox12.Text
num11 = TextBox13.Text
num7 = num1 / (0.25 * 3.14 * 0.6 * 0.6) / 3600
num8 = num2 / (12.7 * 31) / 3600
TextBox11.Text = 0.021 * (num7 * 0.6 / 81.2 / 10 ^ (-6)) ^ (0.84) * 0.595 ^ (0.36) * (0.615 / 0.61) ^ (0.25) * 0.0699 / 0.6
TextBox12.Text = 0.021 * (num8 * 0.6 / 79.3 / 10 ^ (-6)) ^ (0.84) * 0.7 ^ (0.36) * (0.7 / 0.7) ^ (0.25) * 0.0562 / 0.6
TextBox13.Text = num9 * num10 / (num9 + num10)
错误就是TextBox13.Text = num9 * num10 / (num9 + num10)这一行，我有很多这个样子的语句，都是个错误，是最前面定义的错误，还是其他的？
求各位大神帮帮我这个菜鸟吧，谢谢各位大神了！

Answer 1

 Dim num11 As double
Dim num8 As double
Dim num7 As double
Dim num9 As double
Dim num11 As double
Dim num10 As double
num9 = CDbl(TextBox11.Text)
num10 = CDbl(TextBox12.Text)
num11 = CDbl(TextBox13.Text)
num7 = num1 / (0.25 * 3.14 * 0.6 * 0.6) / 3600
num8 = num2 / (12.7 * 31) / 3600
TextBox11.Text = CSTR(0.021 * (num7 * 0.6 / 81.2 / 10 ^ (-6)) ^ (0.84) * 0.595 ^ (0.36) * (0.615 / 0.61) ^ (0.25) * 0.0699 / 0.6)
TextBox12.Text = CSTR( 0.021 * (num8 * 0.6 / 79.3 / 10 ^ (-6)) ^ (0.84) * 0.7 ^ (0.36) * (0.7 / 0.7) ^ (0.25) * 0.0562 / 0.6)
TextBox13.Text =  CSTR(num9 * num10 / (num9 + num10))