A food truck parks across the street from a taco stand. The food truck also carries tacos

Question:

A food truck parks across the street from a taco stand. The food truck also carries tacos but the price is 20% lower than the stand. The cross price elasticity is 1.5. Are the food truck and taco stand selling substitute or complement products?
Given the cross price elasticity, what can the taco stand expect?
How would you know if there was an autocorrelation problem in the regression for the answer to the question above?