Well, first of all - if you know in advance you're going to struggle, don't leave it to the last minute.
If you didn't think you would find it so difficult and you left it to the last minute (which you really shouldn't have) then you should contact your teacher, or whoever is in charge of the assignment and ask them for help, and an extension if you feel like you need one. Teachers are there to help you, they want you to learn and succeed - not fail. Side note: You really should ask for help whenever you don't understand something as soon as possible - don't just wait until you realise you need that knowledge and don't have it.
Under no circumstance should you ever have the need to utilise these 'services' that write essays/complete assignments for you. Apart from being fraudulent a large majority of the time, and even ignoring the ethical concern of handing in someone else's work as your own (plagiarism is very bad!), by delegating your assignment to these 'service providers' you essentially miss out on learning the skills you'll need to complete similar tasks in the future, as well as wasting the opportunity to revise and expand your knowledge of the relevant topics. You also won't receive feedback on your own performance, which is critical to identifying your strengths and weaknesses. You won't get better if you don't know what areas you need to focus on more.
Sure, the idea of "instant solutions" sounds appealing but it's really not worth it.