How to get last line, maximum and min value on string that was stored on richtextbox

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VB

i have this value on my richtextbox

1,2 
3,4 
7,1
5,6
1,5
6,7
...........

there is no specific length or line that it will stop but it will stop when i press the button to stop sending data i would like to ask how to select or get the specific data
i need to get the max,min and last line i hope you understand my English thank you in advance!

What I have tried:

i tried this one

Dim mx As String
Dim mn As String
Dim lv As String
        mx = resulttxt1.Lines.Max.ToString 'it work and i get 7,1 
        mn = resulttxt1.Lines.Min.ToString ' this dont work anymore