Instead of \( (x-9)(x-5) \), what should this equation be? \[ (u-5)(u-9) \] \( \left(x^{2}-5\right)\left(x^{2}-9\right) \)

To clarify the equation given in the question, instead of using the expression \( (x-9)(x-5) \), you should use \( (u-9)(u-5) \), which applies a slight variable change from \( x \) to \( u \). Furthermore, if you were looking for a similar expression that matches the structure of \( \left(x^{2}-5\right)\left(x^{2}-9\right) \), the equations don't directly correlate since they serve different polynomial structures. The expression \( (u-9)(u-5) \) is a quadratic, while \( \left(x^{2}-5\right)\left(x^{2}-9\right) \) deals with squared terms, which expands into a quartic polynomial. It’s crucial to keep the expression's nature consistent when making substitutions!