hey is it true if you fail maths sac unit 3 then cant do unit 4..........
If you fail UNIT 3, then you cannot achieve a UNIT 3/4 sequence in the one year. You can fail a UNIT by not demonstrating a minimum competence at the key skills defined by the study design for that unit. You could then repeat that "failed" UNIT in another year yet still complete unit 4 in this year. However, the score for that broken sequence would possibly not count toward your ENTER unless you completed both units 3&4 in the one year.
But "failing" any SAC should not necessarily mean you have failed a UNIT. If that SAC was used to demonstrate your competence in some skills and you did not do so, then the school should have some procedure where you could separately demonstrate those competencies before the end of the semester.
See your VCE Coordinator ASAP if the issue is applicable to your first semester.