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First, what do you want to solve? It's a function, not an equation. Do you want to solve loesung(lambda_z, s_k) = 0? Do you want to compute loesung(5, 4)? Second, I believe that the line defining loesung will overwrite the previous definition of lambda_z, making it a symbolic variable instead of just being 5.

So perhaps:

\begin{sagesilent}

lambda_za = 5  # note the 'a' at the end
s_ka = 4
loesung (lambda_z, s_k) = lambda_z * s_k

\end{sagesilent}
$ \sage {loesung }$

This will print out 20:    
$ \sage {loesung(lambda_za, s_ka) }$