3^x=4 3^y=6
9^(x-y)+27^(x-y)
=(3²)^(x-y)+(3³)^(x-y)
=3^(2x-2y)+3^(3x-3y)
=(3^x)²/(3^y)² +(3^x)³/(3^y)³
=4²/6² + 4³/6³
=4/9+ 8/27
=20/27