A level further mathematician here (who's never done an enrolment test, so is not an expert), but my guess:
Making X the subject of the equation - something a lot of the people I tutor struggle with.
Factorising - an oldie, but a goodie.
Adding and subtracting algebraic expressions - basic stuff which is useful.
Simplifying algebraic fractions - always useful, and algebraic fractions show up a lot in the A level.
Dividing algebraic equations by other algebraic equations: ie x^3 + 2x^2 - x - 2 divided by (x-1).
Remainder theorem and factor theorem - if α is a root of the equation, f(α) = 0.
For instance:
Unparseable latex formula:[br]$f(x) = x^3 + 2x^2 - x - 2 = 0$//[br]As $f(1) = 1 + 2 - 1 - 2 = 0$ then[br]$1$ is a root of $f(x)$ so that when $f(x)$ is divided by $(x-1)$ the remainder is 0.[br]
That was a rubbish explanation - sorry.
Quadratic formula - always good to know, and useful:
Unparseable latex formula:[br]$\frac{-b \pm \sqrt{b^2 -4ac}}{2a}$[br]
Edit: Anyway, this is just my guess of what could show up. Have you been given a revision list?
Also, maybe you could also do algebra in the context of volumes and surface areas, or basic differentiation, but I have no idea.