can someone please explain how to get -2a+8b-3c from this

step by strep
first the a variable is 2-4= -2 =-2a
cuz itrs the same variable on both ''sides' but even with no () it would be the same
secondly the b varible is 3-5 = -2 =-2b nnnonono the negative in front of the() 'second pack is null by the negative b so - - = +
so scondly from this we got 3--5=8 remember -- gives + so its +8b
thirdly the negative right before variable dosent null with - infront of the pack meaning -c witch is in fact -1c - so -1-2 = -3 so its -3c
when last combined in order -2a +8b -3c

Atariseth:

step by strep
first the a variable is 2-4= -2 =-2a
cuz itrs the same variable on both ''sides' but even with no () it would be the same
secondly the b varible is 3-5 = -2 =-2b nnnonono the negative in front of the() 'second pack is null by the negative b so - - = +
so scondly from this we got 3--5=8 remember -- gives + so its +8b
thirdly the negative right before variable dosent null with - infront of the pack meaning -c witch is in fact -1c - so -1-2 = -3 so its -3c
when last combined in order -2a +8b -3c

the rules is simple you have to distribute the ''-'' before the ''()'' to eatch variable indidualy since its minus alll in '()''

(2a+3b-c)-(4a-5b+2c)

I'd suggest that you get rid of the brackets first, which I know is what guest just said.
I'd just look at it a bit differently

There is an invisable 1 between the minus sign and the second bracket - put that in and you have

(2a+3b-c) -1 (4a-5b+2c)
now expand the second bracket by mulltiplying everything by -1 and you get
(2a+3b-c) -4a +5b - 2c
Just like guest said - when multiplying by -1 all the signs inside the bracket swap.
tThe first brack has always been unnecessary
so you have

2a+3b-c -4a +5b - 2c
now you can just collect like terms.