Choose View > Zoom In, twice, to zoom in to the artwork.
Select the Knife tool ( ). Move the pointer just above the top point of the selected triangle.
Press the Caps Lock key to turn the Knife tool pointer into crosshairs ( ).
The crosshairs pointer is more precise and can make it easier to see exactly where you begin
cutting.- Press and hold Option+Shift (macOS) or Alt+Shift (Windows), and drag down, all the way
across the shape to cut it into two, in a completely straight line. Release the mouse button and
then the keys.