Using what you learned in this section, determine the type of encoding used in the string you got at previous exercise, and decode it. To get the flag, you can send a 'POST' request to 'serial.php', and set the data as "serial=YOUR_DECODED_OUTPUT".
I was do it and find a flag with encode using base64 the flag is 7h15_15_a_s3cr37_m3554g3
but I got an error (wrong flag)
Please help me solve this