Practice, practice, practice.
Mathmatics, particularly the use of it, is a skill all physicists need to develop. You need have a good go at those question sets and come in next week with any issues.
Whenever you take on any maths problem the first trick is to identify which formula to use (that’s why knowing them is so important). Identify what you need to work out and what varable the question has told you. The formula will link these together. Rearrange, numbers in, check it makes sense and you’re done.
In case you are interested you can look at the PAT papers (the difficult questions we projected) at: http://www2.physics.ox.ac.uk/study-here/undergraduates/applications/physics-aptitude-test-pat.