Linear regression proof question:

In a typical multiple linear regression model where n and X2 are two non-ra ndom regressors, the expected value of the response variable y given X1 and x2 is denoted by My l x1, x2). Build the model for the scenario where one of the regressors my: change the value of Efy | x1, x2) differentially as the value of the other regressor changes. HOW can such a potential change be assessed statistically