The work of two students is shown for the problem: What is the slope of a line that is perpendicular to a line that passes through (–24, 15) and (18, 13)?


Student A Student B
m equals the quantity 13 minus 15 end quantity over the quantity 18 minus negative twenty-four end quantity equals negative 2 over forty two equals negative 1 over twenty one. m equals the quantity 13 minus 15 end quantity over the quantity 18 minus negative twenty four end quantity equals negative 2 over forty two equals negative 1 over twenty one.
The slope of the perpendicular line is 21. The slope of the perpendicular line is –21.
Part A: Which student has the correct slope of the perpendicular line? (2 points)

Part B: Explain how the answer to Part A is correct. (2 points)