Question 52092
LETS TRY A DIFFERENT APPROACH AND DO IT GRAPHICLY 

PLANK1+PLANK2+PLANK3 IS SHORT BY 1 FOOT OR -1
PLANK1+PLANK2+PLANK3+PLANK4 IS 2 FEET LONG OR +2 THE DISTANCE BETWEEN -1&+2=3
THEREFORE THE LENGTH OF THE PLANK MUST BE 1 FOOT + 2 FEET=3 FEET LONG

SORRY FOR THE MISINFORMATION. I MUST HAVE HAD A LAPSE BECAUSE I KNEW THE ANSWER HAD TO BE 3 FEET BUT DIDN'T PAY ATTENTION TO THE ANSWER I GOT.