The ACT is 25% geometry, 6% trigonometry and 5% analytic geometry, so you better review your geometry! What makes the test challenging is how many subjects are included on the test, so to get a high score you have to invest time in rarely used facts about absolute value, logarithms, matrices and sequences & series. So, get LOTS of practice by doing several free practice tests and get some tutoring to help kick start your studying.