18 year old, just finished A-levels in humanities (When 18 years old, like a high school diploma). Got a 7 (A) in maths in my GCSEs (When 16 yrs old, like a lower High school diploma). I want to do a bit of maths and get back into to it to improve cognitive skills and reasoning and as well as learning new knowledge and giving me something to do. Where do I start? Do I teach myself A-level maths?