a, \(\frac{2^{15}.\left(-9\right)^4}{-6^3.8^3}=\frac{2^{15}.\left(-3.3\right)^4}{-\left(2.3\right)^3.\left(2^3\right)^3}=\frac{2^{15}.3^4.3^4}{-2^3.3^3.2^9}=\frac{2^{15}.3^8}{-2^{12}.3^3}=\frac{2^3.3^5}{-1}=-8.243=-1944\)
b, \(\frac{8^{10}+4^{10}}{8^4+4^{11}}=\frac{2^{30}+2^{20}}{2^{12}+2^{22}}=\frac{2^{20}\left(2^{10}+1\right)}{2^{12}\left(1+2^{10}\right)}=\frac{2^{20}}{2^{12}}=\frac{1}{2^8}=\frac{1}{256}\)