Thanks for your Answer. I tried it with your Code but it doesnt work for me.

Any idea what i made wrong again? Do you have also an idea what i Need to type in to get an Expression for m =…… with variables so that i have an General Expression.

What means this " ` " Symbol ?

Is a numeric value with MachinePrecision see for more info.

With correct syntax:

  $Version
   (* "12.0.0 for Microsoft Windows (64-bit) (April 6, 2019)" *)


 ClearAll["Global`*"]
 Tx = 68.4; T9 = 25.4; T1 = 90; L = 0.35; x = 0.05;
 NSolve[(Tx - T9)/(T1 - T9) == Cosh[m (L - x)]/Cosh[m L], m, Reals]
 (*{{m -> -8.22068}, {m -> 8.22068}}*)


 Solve[(Tx - T9)/(T1 - T9) == Cosh[m (L - x)]/Cosh[m L], m, Reals]
 {{m -> -8.22068}, {m -> 8.22068}}

Solve gives a error message because Solve command works better with exact values.

ClearAll["Global`*"]
Tx = 684/10; T9 = 254/10; T1 = 90; L = 35/100; x = 5/100;
Solve[(Tx - T9)/(T1 - T9) == Cosh[m (L - x)]/Cosh[m L], m, Reals]
%//N
(*{{m -> -8.22068}, {m -> 8.22068}}*)

 ClearAll["Global`*"]
  Solve[(Tx - T9)/(T1 - T9) == Cosh[m (L - x)]/Cosh[m L], m, Reals]
 (*General solution is impossible*)