$$\sqrt{(64*а^8)(b^{10})(c^9)} = \sqrt{(2^6*а^8)(b^{10})(c^9)} =$$$$= (2^6*а^8*b^{10}*c^9)^\frac{1}{2} = 2^\frac{6}{2}*а^\frac{8}{2}*b^\frac{10}{2}*c^\frac{9}{2} =$$$$= 2^3*а^4*b^5*c^{4\frac{1}{2}} = 2^3*а^4*b^5*c^4*\sqrt{c}$$