\(4^5\cdot9^4-2.6^9=\left(2^2\right)^5.\left(3^2\right)^4-2.\left(2.3\right)^9=2^{10}.3^8-2.2^9.3^9=2^{10}.3^8-2^{10}.3^9=3^8-3^9=-13122\)
\(2^{10}.3^8+6^8.20=2^{10}.3^8+\left(2.3\right)^8.2^2.5=2^{10}.3^8+2^8.3^8.2^2.5=2^{10}.3^8+2^{10}.3^8.5=2^{10}.3^8.\left(1+5\right)=2^{10}.3^8.6=2^{10}.3^8.2.3=2^{11}.3^9\)