原式=(a+2b-3a+b)/(4a²-9b²)+2/(2a+3b)
=-(2a-3b)/(2a+3b)(2a-3b)+2/(2a+3b)
=-1/(2a+3b)+2/(2a+3b)
=1/(2a+3b)
=1/(2+9)
=1/11