\[\boxed{\ \mathbf{471.}}\]
\[1)\ 54a - 39a = (54 - 39)a =\]
\[= 15a\]
\[a = 26:\]
\[15 \cdot 26 = 390.\]
\[2)\ 18m - 5m + 7m =\]
\[= (18 - 5 + 7)m = 20m\]
\[m = 394:\]
\[20 \cdot 394 = 7880.\]
\[3)\ 19z - 12z + 33z - 19 =\]
\[= (19 - 12 + 33)z - 19 =\]
\[= 40z - 19\]
\[z = 82:\]
\[40 \cdot 82 - 19 = 3280 - 19 =\]
\[= 3261.\]