The line tool can be placed in any cell and can be extended to any length. First, tap (select) the cell where you want to place the line, then tap the line symbol on the keypad. Next, select the line on the canvas by tapping it. Try zooming in (pinch out) if you have problems selecting the line. Once the line is selected tap and hold (long tap) the dot until you see the solid line change to a dotted line. Then drag the dot to make the line longer. This behavior works on the square root and long division symbols as well.