Question 815861
First solve for y 

{{{5x-7y=21}}} 
{{{-7y=-5x+21}}} 
{{{7y=5x-21}}} 
{{{y = (5/7)x - 3}}} 

Since we know m = slope and that when this equasion is solved for y it will be in general y= mx + b

Accordingly, 5/7 = m, thus slope ie equal to 5/7.