Question 1064497
The smaller (upper) triangle has sides that are proportional to the larger (outermost) triangle.  Let's set up a proportion:

    {{{ (3x-20)/((3x-20)+56) = (9/(9+72)) }}}
 Noting that   {{{ 9/(9+72) = 9/81 = 1/9 }}}
 Let's us simplify to:
    {{{  (3x-20) / (3x+36) = 1/9 }}}
    {{{  9*(3x-20) = 3x+36 }}}
    {{{   27x - 180 = 3x+36 }}}
     {{{  24x = 216 }}}
    {{{  x = 216/24 = highlight(9) }}}

—
Check:   (3(9)-20) / (3(9)-20+56) = 7/63 = 1/9  (= 9/81 = 9/(72+9), so its ok)