Question 611036
Restating your original problem just to make it easier to read:
{{{ sqrt(8)*sqrt(2) - 7 }}}

That can be simplified by first prime factorizing 8, which results in the prime factors of 2 * 2 * 2. So now the problem can be rewritten by substituting the prime factors for 8:
{{{ sqrt(2*2*2)*sqrt(2) - 7 }}}

You can split those all out separately:
{{{ sqrt(2)*sqrt(2)*sqrt(2)*sqrt(2) - 7 }}}

Since {{{sqrt(2)*sqrt(2) = 2}}}, the above problem can be simplified as:
{{{ 2*2 - 7}}}

Which can then be further simplified as:
{{{-3}}}