`3a^2 - 4[2b-3b(b+2)]`

Begin with the inner most bracket and simplify:

`3a^2 -4[2b-3b^2 - 6b]`

Note how the -3b has been distributed between the (b+2) and note the symbols have changed to negative.

Now distribute the -4 into the bracket :

`3a^2 - 8b + 12b^2 + 24b`

Again care with the symbols.Now put like terms together and simplify:

`3a^2-8b+24b +12b^2`

`= 3a^2+16b+12b^2`