\[3a^{2} + 7a - 5 + 3a^{2} + 1 =\]
\[= 6a^{2} + 7a - 4.\]
\[3a^{2} + 7a - 5 - \left( 3a^{2} + 1 \right) =\]
\[= 3a^{2} + 7a - 5 - 3a^{2} - 1 =\]
\[= 7a - 6.\]