Hacked by malicious code

i have my main website hacked by malicious code as attached below, however, only 1 of my website become blank page frequently.

May i know how to solve it?

Comment from my web hosting server as below:
As I mentioned before, all your php files in your hosting account is infected, you must update all sites in your hosting account, not only [domain].com (all scripts, every single php files).
You can ask your programmer/web developer to open any php file in your hosting account, it is injected at first line of php files.

However, i am not developer, i am unable to change it every single php file??

Please help. Thanks.