Summarize:

The formula states that d=(x2−x1)2+(y2−y1)2{\displaystyle d={\sqrt {(x_{2}-x_{1})^{2}+(y_{2}-y_{1})^{2}}}}, where d{\displaystyle d} equals the distance of the line, (x1,y1){\displaystyle (x_{1},y_{1})} equal the coordinates of the first endpoint of the line segment, and (x2,y2){\displaystyle (x_{2},y_{2})} equal the coordinates of the second endpoint of the line segment. These might already be given. If not, count along the x-axis and y-axis to find the coordinates.  The x-axis is the horizontal axis; the y-axis is the vertical axis. The coordinates of a point are written as (x,y){\displaystyle (x,y)}. For example, a line segment might have an endpoint at (2,1){\displaystyle (2,1)} and another at (6,4){\displaystyle (6,4)}. Be careful to substitute the values for the correct variables. The two x{\displaystyle x} coordinates should be inside the first set of parentheses, and the two y{\displaystyle y} coordinates should be inside the second set of parentheses. For example, for points (2,1){\displaystyle (2,1)} and (6,4){\displaystyle (6,4)}, your formula would look like this: d=(6−2)2+(4−1)2{\displaystyle d={\sqrt {(6-2)^{2}+(4-1)^{2}}}}
Set up the Distance Formula. Find the coordinates of the line segment’s endpoints. Plug the coordinates into the Distance Formula.