I was given the following task in a course:

You examine the influence of the two variables QS and TR on the variable WF. You have estimated the following relationship:

WF= - QS - TR + 4QS • TR + ε

WF is your dependent variable. The variables QS and TR are your independent variables. ε is the residual.

1. What is the marginal effect of QS on WF when TR has the value 0?

2. What is the marginal effect of TR on WF when QS has the value 0?

3. what is the marginal effect of QS on WF when TR has the value -2?

4. what is the marginal effect of TR on WF when QS is -2?

Unfortunately, I have no idea how to start. Could someone please help me?