The**slope of a line**is a measurement of the steepness and direction of a nonvertical line. When a line rises from left to right, the slope is a positive number. Figure 1(a) shows a line with a positive slope. When a line falls from left to right, the slope is a negative number. Figure 1(b) shows a line with a negative slope. The*x*‐axis or any line parallel to the*x*‐axis has a slope of zero. Figure 1(c) shows a line whose slope is zero. The*y*‐axis or any line parallel to the*y*‐axis has no defined slope. Figure 1(d) shows a line with an undefined slope.

You are watching: Slope of a line parallel to x axis

**Figure 1 **Different possibilities for slope of a line.

If*m*represents the slope of a line and*A*and*B*are points with coordinates (*x*l,*y*1) and (*x*2,*y*2) respectively, then the slope of the line passing through*A*and*B*is given by the following formula.

*A*and*B*cannot be points on a vertical line, so*x*1and*x*2cannot be equal to one another. lf*x*1=*x*2, then the line is vertical and the slope is undefined.

**Example 1:**Use Figureto find the slopes of lines*a, b, c*, and*d.*

**Figure 2 ***Finding the slopes of specific lines.*

a. (a) Line*a*passes through the points (−7, 2) and (−3, 4).

b. (b) Line*b*passes through the points (2, 4) and (6, −2).

c. (c) Line*c*is parallel to the*x*‐axis. Therefore,*m*= 0.

d. (d) Line*d*is parallel to the*y*‐axis. Therefore, line*d*has an undefined slope.

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**Example 2:**A line passes through (−5, 8) with a slope of 2/3. If another point on this line has coordinates (*x*, 12), find*x.*

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