The expression \(-5 \sqrt{x}\) involves the square root of \(x\). For the square root function to produce real number results, the value of \(x\) must be non-negative. This leads us to the following condition:
\[ x \geq 0 \]
Therefore, the expression \(-5 \sqrt{x}\) makes sense for all \(x\) in the interval:
\[ [0, \infty) \]
In summary, the values of \(x\) that make the expression \(-5 \sqrt{x}\) valid are all non-negative values of \(x\).