\[(5 - x)^{2} - x(2,5 + x) = 0\]
\[25 - 10x + x^{2} - 2,5x - x^{2} = 0\]
\[- 12,5x = - 25\]
\[x = - 25\ :( - 12,5)\]
\[x = 250\ :125\]
\[x = 2.\]
\[Ответ:2.\]