The linear regression equation y = 4.2x + 120,000 models the relationship between the amount of money x a...

The linear regression equation y = 4.2x + 120,000 models the relationship between the amount of money x a company spends on advertising, in dollars, and their revenue y, also in dollars.
What is the rate of change in the regression equation y = 4.2x + 120,000? Explain the meaning of this number.