The extension 1 knowledge is vital. I suppose a bridging course might bring you up to speed (quite a bit of the Ext 1/2 calculus is retaught briefly throughout MATH1151). However, what is more important is having the ability to understand the concepts. If you had the ability to (so far as you can tell) Ext 1 or 2 but chose to do 2U for some reason then you are probably capable of picking it up and doing decently. If, though, you did 2U because you were told or know it was all you could manage conceptually, then actuarial studies is probably not for you. Let me know if you have any questions