4. Consider the consumer in (2), but ignore the information in (3). Holding prices fixed: a. Derive an expression for the Engel Curve for ????". Hint: use the demand function to get a relationship between ???? and ????? (the optimal demand for ???? ), holding ???? ""! and ????" fixed. ? b. Plot this Engel curve (put ???? on the vertical axis, and ????" on the horizontal axis). c. On a different graph, plot this consumer’s demand curve for ????", holding ????! and ???? fixed. Label the axes.