Answer:
When it comes to 18 all I can say is that the cities are near the border in the warmer, more fertile areas of the country. A lot of the area is still cold, plus the closeness to the USA adds an economic advantage. So for #18, it's because of the fertile growing land and proximity to its closest trading partner. For #19, I'd say they're close to the ocean for one. Mexico trades oil on a global scale, so the richest places are on the ocean/seas. This differs from Canada who trades a lot with the USA, so their trade lcoations are close to the USA. Meanwhile Mexico uses considerably more sea trade, therefore their better cities are closer to the ocean/seas.
Step-by-step explanation:
AP Human Geography and a bit of logic. it's a rough answer and I'm not sure it's right but I feel like it answers the questions.