Answer
$\color{blue}{17ab^2\sqrt{2ac}}$
Work Step by Step
Simplify each radical:
$=7\sqrt{25a^2b^4(2ac)}-2ab\sqrt{81b^2(2ac)}
\\=7\sqrt{5^2a^2(b^2)^2(2ac)}-2ab\sqrt{9^2b^2(2ac)}
\\=7(5ab^2)\sqrt{2ac} - 2ab(9b)\sqrt{2ac}
\\=35ab^2\sqrt{2ac}-18ab^2\sqrt{2ac}$
Combine like terms to obtain:
$=(35-18)ab^2\sqrt{2ac}
\\=\color{blue}{17ab^2\sqrt{2ac}}$
