Answer :
You couldn't get a direct answer from what the information has been giving, but the formula would be WxL(2.5). The area of the window, plus the extra area needed to touch the floor.
a= top(length) x bottom(width)
= 4x 2.5
= 10 feet
Therefore Leah should buy a 10 feet blanket
...... Am I wrong? feel free to correct me.