never it too late for studying.
Get a good number theory from amazon....and start the job.
Tom Apostol's analytic number theory book is a classic in this field.
I don't think anything other than the most basic number theory is taught in my country and therefore I am wondering where I can learn to solve problems such as: Find all such that
I'm seeing high-schoolers post these and in my country we don't learn these in high school or even college unless you major in maths.
I can't find anything on it at Khan Academy...Not sure where to look!