\[\boxed{\mathbf{7}\mathbf{.}}\]
\[\textbf{а)}\ 34 + 7 = 41\]
\[3 + 29 = 32\]
\[41 - 2 = 39\]
\[52 - 3 = 49\]
\[\textbf{б)}\ 25 + 17 + 0 = 42 + 0 = 42\]
\[49 + 0 + 34 = 49 + 34 = 83\]
\[73 - 18 - 0 = 55 - 0 = 55\]
\[91 - 0 - 45 = 91 - 45 = 46\]