A Dodge Neon and a Mack truck leave an intersection at the same time. The Neon heads east at an average speed of 30 mph, while the truck heads south at an average speed of 40 mph. Find an expression for their distance apart d (in miles) at the end of t hours.

At first sight, I quickly decided to set up my table for D = rt. I also know that moving vehicles starting at the same location and traveling in opposite directions have equal distances if a TOTAL DISTANCE is given, which is clearly not the case here. However, this led nowhere. The question is asking for an expression for their distance apart denoted by d in miles. Seeking one or two hints.