the answer is 5 cm..
Refer to the image..
Draw a line parallel to CF from M..
Therefore K is the midpoint of AC...
Draw a line parallel to AB from point P..it will also meet at K..
Now consider the triangle KPM
where KP is 3cm..(1/2 of AB)(From midpoint theorem)
KM=4cm(1/2 of CF)(From midpoint theorem)
Therefore PM=√32+42
=5..