1.
Croft, T. & Davison, R. Foundation maths. (Pearson, 2020).
2.
Elementary Algebra Textbook. http://mathrev.redwoods.edu/ElemAlgText/.
3.
Mathematics Enhancement Programme. http://www.cimt.org.uk/projects/mepres/alevel/alevel.htm.
4.
Underground Mathematics. https://undergroundmathematics.org/.