Question 534786
Suppose I say {{{ z = -3x }}}
Then
{{{ 6^z = 15 }}}
I know that
{{{ 6^1 = 6 }}}
{{{ 6^2 = 36 }}}
So, {{{z}}} is somewhere between {{{1}}} and {{{2}}}
and, 
{{{ 1 = -3x }}}
{{{ x = -(1/3) }}}
{{{ 2 = -3x }}}
{{{ x = -(2/3) }}}
so, {{{x}}} must be between -(2/3) and -(1/3)
You could find the answer by trying
{{{ 6^1.1 }}}
{{{ 6^1.2 }}}
etc
on a calculator until you get very, very close to 15