start from the inside and work your way out.
we would start with simplifying (2a-4a) first, giving us (-2a) instead of the previous.
(4a-(-2a)) would give us a positive 6a.
(3a-6a) would give us a (-3a)
a-(-3a) would leave us with a simplified 4a.
that's the answer, hope that helped!! xx