3^(3x+5)-27^(x+1)=648
3^(3x+5)-3^(3x+3)=648
3^(3x+3)×(3²-1)=648
3^(3x+3)×8=648
3^(3x+3)=81
3^(3x+3)=3^4
3x+3=4
3x=1
x=1/3