Why did I get warned for giving points away?

Which line is parallel to the line that passes through the points (2, –5) and (–4, 1) a. y=-x+5 b. y=-2/3x+3 c. y=2/3x-2 d. y=x+5

Aclassmate thinks that solving a system by graphing gives an exact answer when the lines appear to cross at a grid point, but only an approximate answer when they don't. explain why this isn't true.

Find the average rate of change for the given function drom x=1 to x=2