Question 1110286
Please be aware when writing expressions, here and elsewhere:  
24-20/2  (=14)  should be  written (24-20)/2  (=2).

Check out the lessons area, some tutors have posted lessons on order-of-operations.

—

I think the common rule for [a,b,c]  is   {{{ c=sqrt(a-b) }}}

So for [69,?,8],  using b instead of ? :    

{{{  8 = sqrt(69-b) }}}
 {{{ 64 = 69 - b  }}}
 {{{ highlight( b  = 5 ) }}}