27.已知A={x∣x2−2x−24<0}A=\{x\vert x^{2}-2x-24<0\}A={x∣x2−2x−24<0},B={x∣x+34−x⩾0}B=\left\{{x\vert {\frac{x+3}{4-x}\geqslant 0}}\right\}B={x∣4−xx+3⩾0},C={x∣a<x<a+6}C=\{x\vert a<x<a+6\}C={x∣a<x<a+6}. (1)求A⋂(∁RB)A\bigcap (\complement _{R}B)A⋂(∁RB); (2)若B⋃C=BB\bigcup C=BB⋃C=B,求实数aaa的值.