\[\boxed{\mathbf{3.}\mathbf{51}}\]
Пояснение.
Решение.
\[\textbf{а)}\ 43 + m = 43\]
\[m = 0.\]
\[\textbf{б)}\ n + 25 = 25\]
\[n = 0.\]
\[\textbf{в)}\ 57 - x = 57\]
\[x = 0.\]
\[\textbf{г)}\ 108 - y = 0\]
\[y = 108.\]
\[\textbf{д)}\ z + 0 = 0\]
\[z = 0.\]
\[\textbf{е)}\ 0 - t = 0\]
\[t = 0.\]
\[\textbf{ж)}\ a - a = 0\]
\[a - любое\ число.\]
\[\textbf{з)}\ c + c = 0\]
\[c = 0.\]