A retail store finds that its monthly sales revenue $S$ (in thousands of dollars) is related to its advertising spend $x$ (in thousands of dollars) by the function $S(x) = -2x^2 + 20x + 50$. If advertising costs \1$thousand per unit of$x$, the profit function is$P(x) = S(x) - x$. What advertising spend$x$ (in thousands of dollars) maximizes the store's profit?