6. (20 5) ) David always consumes good x and good y together, and he always consumes the two goods at the ratio x/y = 3/4. (1) Write down David's utility function and find out his demand functions for good x and good y. (2) Write down the equation for David's Engel curve of good x. (3) The price of x is $4 and the price of y is $7. David's income is $600. Find out David's demand bundle and calculate the slope of David's Engel curve for good x at these prices. (4) If David's income doubles and prices remain unchanged, find out David's demand bundle. (5) Draw David's Engel curve for good x. Label all the necessary numbers on the graph