Then the ordinate of point M is 2m, and the ordinate of point N is 3m.

So MN=3m? 2m= 1m，

∴MNPM= 1m2m= 12，

So the answer is:12;

(2) As shown in figure 1, it is easy to know M(m, 3m), N(m, 2m), P(m, 0),

∴PM=3m and PN=2m, so d1= pm-pn = m.

As shown in Figure ②, it is easy to know that M(m, 3m), N(m2m), P(m, 0),

∴PM=3m,PN=2m，

Therefore, D2 = pm-pm =1m.

(3) As shown in Figure ③, we can get M(m, m2-4m), N(m, m2-3m) and P(m, 0) according to the topic.

∵m>0,∴op=m,pm=|m2-4m|=m|m-4|,pn=|m2-3m|=m|m-3|,mn=(m2-3m)-(m2-4m)=m.

Therefore, m|m-4|=m or m|m-3|=m,

∴m=5 or m=3 (irrelevant),

Either m=2 or m=4 (irrelevant).

According to the meaning of the question, we can get a (3 3,0) and b (4 4,0).

∴ When m=5, PA=2, PB= 1, PN= 10, PM=5,

At this time, the s quadrilateral abmn = s △ pan-s △ pbm =12×10× 2-12× 5×1= 7.5;

When m=2, PA= 1, PB=2, PN=2, PM=4,

At this time, the S quadrilateral ABMn = S △ PBM-S △ Pan =12× 4× 2-12× 2×1= 3.