Suppose you estimate a linear regression model of ticket sales ($S$) as a function of consumers' income ($I$) in \$1,000, income squared ($I^2$), and price of the ticket ($P$):

$$S = \beta_0 + \beta_1 I + \beta_2 I^2 + \beta_3 P + \varepsilon$$

What is the marginal impact of an increase in income on ticket sales?