N-4-9 when simplifying

Hi, can anyone show me how to do this problem? 100 points for this. in advance

Find the slope of the line passing through a pair of points

Asmall business produces and sells balls. the fixed costs are $20 and each ball costs $4.32 to produce. each ball sells for $8.32. write the equations for the total cost, c, and the revenue, r, then use the graphing method to determine how many balls must be sold to break even.