2x+y=b,x-3y=1
x=(3b+1)/7,y=(b-2)/7
x+3y=(3b+1)/7+3(b-2)/7=(6b-5)/7
14-(x+3y)^2-4(3y-x)^3
=14-[(6b-5)/7]^2+4(x-3y)^3
=14-(36/49b^2-60/49b+25/49+4
=857/49+60/49b-36/49b^2