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5a-{-[(-3a+2b)-4b]+[-7a-(a-b)]-3b}

First get rid of the parenthasese around '-3a +2b'

5a-{-[-3a+2b-4b]+[-7a-(a-b)]-3b}

now distribute the '-'

5a-{3a-2b+4b+[-7a-(a-b)]-3b}

now distribute the '-' in front of (a-b)

5a-{3a-2b+4b+[-7a-a+b]-3b}

now get rid of the brackets

5a-{3a-2b+4b-7a-a+b-3b}

Then distribute the negative

5a-3a+2b-4b+7a+a-b+3b

finally combine like terms

5a-3a+7a+a+2b-4b-b+3b

all the b's cancel and you are left with:

10a

You try the second one and I will check your work.