x+3y-4=0
3y=4-x,
Z=3^x+27^y +3
=3^x+3^3y +3
=3^x+3^(4-x) +3
=3^x+81/3^x +3
>=2*9 +3=21
当且仅当3^x=9,即x=2,y=2/3是Z取最小值21