Answer:  " 
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Explanation:
Assume we have a two-dimension Cartesian plane graph; 
 in which equations of straight lines can be expressed in the form of:
 " y = mx + b " ; 
in which:
 "m" is the slope; as well as the "coefficient" of "x" ; 
 and: "b" is the y-intercept; that is, the value "y" value of the "(x, y)" coordinate at the point which "x = 0" ; that is, where the graph crosses the "y-axis".
Given the ^aformentioned conditions:
 Note that if we select a line that can be written in the format: 
 " y = mx + b " ; then another line that is "perpendicular" to that 
[selected line] will have the slope, "m"; of the value that is the "negative reciprocal value" of the slope, "m" ; of the other line.
Note that is our problem, we select a line—in our given problem: 
 "Line a" —with a slope, "m = 7 " ; 
 which is perpendicular to the slope of "Line b" ; 
 → then: the slope of "Line B" ; would be the "negative reciprocal value"
 of "7" .
 Note: " 
 " ; the recriprocal would be: "
 " ; the recriprocal would be: " 
 " .
 " .
 _____________
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