B-(C-2A)
=B-C+2A
=(3x²+5x+4)-(7x²-6x+8)+2(2x²-9x-11)
=3x²+5x+4-7x²+6x-8+4x²-18X-22
=-7X-26
把x=2/7代入-7X-26
得:-7*(2/7)-26
=-28