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

8b-29a

3a + [-7a - 4(5a - 2b) - 5a]
= 3a + [-7a - 20a + 8b - 5a]
= 3a - 7a - 20a + 8b - 5a
= -4a - 20a - 5a + 8b
= -29a + 8b

8b - 29a is correct. I must note that in modern algebra teaching we usually use nested parentheses rather than brackets or bracing for grouping like this, though. That is how one would have to evaluate it with excel, for instance.

=3*a+(-7*a-4*(5*a-2*b)-5*a)

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

First we are supposed to solve the innermost bracket, so,

3a + [ -7a -20a + 8b - 5a]

then we proceed to the next bracket, grouping the like terms

3a + [ (-7a - 20a - 5a) + 8b ]

solving that we get :

3a + [ - 32a +8b]

Once we poen the bracket,

8b - 29a

therefore,
3a + [ -7a - 4(5a - 2b) - 5a] = 8b - 29a

A very concise and clear presentaion , if I may say so:-
3a + [- 7a - 4(5a - 2b) - 5a ]
3a + [- 7a - 20a + 8b - 5a ]
3a + [8b - 32a ]
8b - 29a