\(810-9\left(5+x\right)=3^6\Leftrightarrow810-9\left(5+x\right)=729\)
\(9\left(5+x\right)=810-729\Leftrightarrow9\left(5+x\right)=81\)
\(5+x=81:9\Leftrightarrow5+x=9\)
\(x=9-5\Leftrightarrow x=4\)
810 - 9(5 + x) = 36
810 - 9(5 + x) = 729
9(5 + x) = 810 - 729
9(5 + x) = 81
(5 + x) = 81 : 9
5 + x = 9
x = 9 - 5
x = 4