Answer: 7 units
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Short explanation:
Plot the points (3,1) and (3,8). Note how they line up vertically. Count the number of vertical spaces between them and you should get 7 units. This only works if the points line up horizontally or vertically.
Or you could subtract the y coordinates to get 8-1 = 7, which is an alternative to graphing and counting the spaces. This is preferred if there is a very large gap.
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Longer method:
Use the distance formula with (x1,y1) = (3,1) and (x2,y2) = (3,8)