The given expression,
${ = \sqrt {220 - 2 \times 15 \times \sqrt {35} } }$
${ = \sqrt {220 - 2 \times 3 \times 5 \times \sqrt 7 \times \sqrt 5 } }$
${ = \sqrt {\underbrace {{{(3\sqrt 5 )}^2}}_{9 \times 5 = 45} + \underbrace {{{(5\sqrt 7 )}^2}}_{25 \times 7 = 175} - 2(3\sqrt 5 )(5\sqrt 7 )} }$
${ = \sqrt {{{(3\sqrt 5 - 5\sqrt 7 )}^2}} }$
${ = |3\sqrt 5 - 5\sqrt 7 | = 5\sqrt 7 - 3\sqrt 5 }$