If a car travels 25 mi /h for t hours. t hen travels 45mi/h for m hours what does the expression 25t+45m represent

Your expression represents the total distant traveled. 25t tells you depending on how long you drove, the distance for that period. Then you also drove 45mi/h for an unknown time as well. So adding them gives you the entire distance driven.  

Horizontal change or the change of x