how to do this?

The formula for permutations is:  _nP_r  =  n! / (n - r)!

 

So:                  ₃₀P₂  =  30! / (30 - 2)!  =  30! / 28!

                                                           =  [ 30·29·28·27·26·...·4·3·2·1 ] / [ 28·27·26·...·4·3·2·1 ]

(which, after cancelling, becomes)     =  30·29  =  870