At first we analyze what kind of file it is using file command on the linux terminal.
On this occasion it shows that it is a Brokenfr: 7-zip archive data, version 0.4
so we unzip it using the command 7z e -y Brokenfr -oextracted_data
In the extracted_data directory we see a file name "not_so_suspicious_file" , in CTF if something is described as not suspicious dig that file more..
Again used the file command to check the file type which is unknown file type only describing data in it.
if we examine the Hex data of it using hexdump -C not_so_suspicious_file we see first 4 digit of hex data has been tampered and rest of the hex data indicates it is a image file.
by using the command hexedit not_so_suspicious_file we fixed the hex codes. for PNG hex header bytes are 89 50 4E 47 0D 0A 1A 0A.
At last we will see the file can be opened as image and we will see the flag there.