In short: Clear the unknown variable first: Clear(T.2)=
The equation you are solving for is evaluated in the current context (all known definitions are substituted). In your FindRoot-Example, the variable T.2 which you are solving for, already has a value (just type T.2 to see it). Thus, after substitution, your equation doesn't contain the variable any more.
This is a typical mistake in equation solving. Always make sure that your unknowns are indeed unknown. This is also explained in my example upstream the topic.
Martin Kraska
Pre-configured portable distribution of SMath Studio: https://en.smath.info/wiki/SMath%20with%20Plugins.ashx