How do I map this plasmid

It is a 12.7 kb plasmid. PstI: 6.8 kb, 5.9 kb
Hind III: 6.5 kb, 6.2 kb
Eco Ri 9.2 kb, 3.5 kb
Pst I + Hind III: 4.8 kb, 4.2 kb, 2.0kb, 1.7 kb
Pst I+ Eco Ri: 5.4kb, 3.8 kb, 3.0kb, 0.5 kb
Eco Ri + Hind III: 6.2kb, 3.5kb, 1.8kb, 1.2 kb
Can you please explain ur steps so I can learn it for my test?

Ah, these things are a big pain if you don't like puzzles.
Let's start with PstI. If you draw a circle and put one of the cut sites at position 0, the other cut site will be at either position 5900 or 6800 (in base pairs). It doesn't really matter which you choose, as the relative positions will be the same no matter what. So, now you have the PstI sites separating the circle into 2 pieces - one 6800 and one 5900 bp long.
Look at the PstI + HindIII pieces now: 4800, 2000, 4200 and 1700 bp each. Hmm - interesting; 4800 + 2000 = 6800, and 4200 + 1700 = 5900. It looks like each HindIII cut splits a PstI fragment in two. So, where do the HindIII sites go? Let's put one site at position 4200 to try out, giving us our 4200 and 1700 bp pieces from the double-cut. We now have to figure out how to put the other between position 5900 and position 0 (aka position 12,700). Well, if we just cut with HindIII alone, we have a 6200 and a 6500 bp piece. If we look at the 4200 double-cut piece, we can only add onto it at one end, right, since going the other way we have a 1700 bp piece. That means our other HindIII cut must be at position -2,000 (aka position 10,700).
OK, what do we have: starting from the beginning, we have a PstI cut site position 0, a HindIII cut site at position 4200, another PstI cut site at position 5900 and another HindIII cut site at position 10,700. If we cut with PstI, we get 2 pieces of 5900 and 6800 bp, right? A HindIII cut gives us 2 pieces of 6200 and 6500 bp. Well, that checks out. Cut with both and we get 4200, 1700, 4800 and 2000 bp, right? Alright - on to EcoRI!