\[\boxed{\mathbf{3.414}}\]
\[\textbf{а)}\ 18n - 12n - 3n = 186\]
\[3n = 186\]
\[n = 186\ :3\]
\[n = 62.\]
\[\textbf{б)}\ 25a - 7a - 8a = 400\]
\[10a = 400\]
\[a = 400\ :10\]
\[a = 40.\]
\[\textbf{в)}\ 2x + 8x - x = 243\]
\[9x = 243\]
\[x = 27.\]
\[\textbf{г)}\ 6y - y + 4y = 234\]
\[9y = 234\]
\[y = 234\ :9\]
\[y = 26.\]