5a+3[2b-3(a-b+2c)-(b+c)]
=5a+3(2b-3a+3b-6c-b-c)
=5a+6b-9a+9b-18c-3b-3c
=(5a-9a)+(6b+9b-3b)-(18c+3c)
=-4a+12b-21c

4x²-[2x-(x²-2x+3)-2(x²-3x)]
=4x²-(2x-x²+2x-3-2x²+6x)
=4x²-2x+x²-2x+3+2x²-6x
=(4x²+x²+2x²)-(2x+2x+6x)+3
=7x²-10x+3