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You have two definitions (with the = sign) of p_ges. One of them is the symbol

p_ges = var('p_ges', latex_name=r'\varphi_{ges}')

and the other one is the right-hand side of some equation p_ges == p_1 + p_2 + p_3 + p_4.

You should probably give this right-hand side a different name, e.g.

p_ges_rhs = p_1 + p_2 + p_3 + p_4

Then the equation you want to solve is

d = solve(p_ges == p_ges_rhs, d_k)

You have two definitions (with the = sign) of p_ges. One of them is the symbol

p_ges = var('p_ges', latex_name=r'\varphi_{ges}')

and the other one is the right-hand side of some equation p_ges == p_1 + p_2 + p_3 + p_4.

You should probably give this right-hand side a different name, e.g.

p_ges_rhs = p_1 + p_2 + p_3 + p_4

Then the equation you want to solve is

d = solve(p_ges == p_ges_rhs, d_k)

d = solve(p_ges_rhs == 0, d_k)