\[(4a - 5b)(5b - 4a) =\]
\[= - (4a - 5b)(4a - 5b) =\]
\[= - \left( 16a^{2} - 40ab + 25b^{2} \right) =\]
\[= - 16a^{2} + 40ab - 25b^{2}.\]