The function \( f(x) = 2x^2 + 6x + 4.5 \) is a quadratic function, which is defined for all real numbers. The domain of a quadratic function is typically all real numbers unless there are restrictions (like division by zero or square roots of negative numbers, which do not apply here).
Therefore, the correct response describing the domain of this function is:
\(-\infty < x < \infty\) (negative infinity is less than x, comma x is less than infinity).