You are given a code:

def main_solution(x):
    return ((x + x) + func_15('rxu'))

Can you predict the output of `main_solution("zxb")` without writing any code? Please reason and put your final answer in the following json format: {"output": <your output>}, where <your output> should be the final string.
user
{"ref_input": {"x": "zxb"}, "ref_output": "zxbzxb\u00abr\u00bb\u00abx\u00bb\u00abu\u00bb", "ref_code": "def main_solution(x):\n    return ((x + x) + func_15('rxu'))", "funcname": "main_solution"}
rule