What is the relationship between product X and Y in this demand curve?
Substitute or complement? Qdx = 80 - 3Px + 2Py - 10I

I won't give you the answer, but I will help you through it so you can solve it yourself. From the equation if the price of y increases does Qx increase or decrease? What would you expect Qx to do if x is a complement of y and y's price increases? How about if x is a substitute for y? Please answer these and we'll be happy to check your answers.

If x is a complement of y and y's price increases, Qx will decrease? If x is a substitute of y, then Qx will increase.

If x is a complement of y and y's price increases, Qx will decrease? If x is a substitute of y, then Qx will increase.

Seems logical. So, what's your answer - is x a complement or substitute for y?

Complement?

Are you guessing? Have you figured out what happens to Qx when Py increases?