I found an alternate solution, but I don't believe this is as clean as the one offered by @ThomasisCoding. In this case I used substitution to eliminate the intermediate variables.

>>> from sympy import simplify, symbols, Eq, solve
>>> inp, err, out, fb, a, f = symbols("inp, err, out, fb, a, f")

>>> out_expr = a * err
>>> out_expr = out_expr.subs(err, inp - fb)
>>> out_expr = out_expr.subs(fb, f * out)
>>> solution = solve(Eq(out, out_expr), out)
>>> print(f"{out} = {solution[0]}")
out = a*inp/(a*f + 1)