d) \( 3 x^{2}-18 x \)

To solve the expression \( 3x^{2} - 18x \), we can factor it. Notice that both terms have a common factor of \( 3x \). Factoring this out gives us \( 3x(x - 6) \). This means the expression can be easily simplified, and we can find the roots by setting it equal to zero, leading to the values \( x = 0 \) or \( x = 6 \). This simple quadratic function can represent various real-world scenarios, such as the trajectory of an object thrown in the air or calculating the area of a rectangular space whose dimensions depend on the value of \( x \). By understanding its factors, we can predict key moments, like maximum height or optimal configuration, making it a powerful mathematical tool!