\[\left( x^{2} - 16 \right)\left( x^{2} - 9x + 20 \right) \geq 0\]
\[x^{2} - 9x + 20 =\]
\[= x^{2} - 4x - 5x + 20 =\]
\[= x(x - 4) - 5(x - 4) =\]
\[= (x - 4)(x - 5)\]
\[(x - 4)^{2}(x + 4)(x - 5) \geq 0\]