\[\left( 7b^{2} - 4b + 2 \right) - \left( 5b^{2} - 3b + 7 \right) =\]
\[= 7b^{2} - 4b + 2 - 5b^{2} + 3b - 7 =\]
\[= 2b^{2} - b - 5\]