What other methods could be used to ensure parallel rails (any method, not just the geometry kinds)?

1. what is the phythaforean theorem? 2.what do we use it for? 1.what is the distance formula ? 2.what do we use it for?

Agreeting card company can produce a box of cards for $7.50. if the initial investment by the company was $50,000, how many boxes of cards must be produced before the average cost per box falls to $10.50?

In the diagram, the ratio of ab to ae is (1/2) . what is the ratio of the length of to the length of ?