Pxy[u_, a_, b_, c_, 
          d_ ] :=  {a*u^3 + b*u^2 + c*u  + d };

Clear[Pm1];  Clear[P0];  Clear[P1];  Clear[P2];

v  =  
        Solve[
{a*(-1)^3 + b*(-1)^2 + 
                c*(-1)  + d   - Pm1 == 
              0, 
d - P0 == 0, 
 
            a + b + c  + d - P1 == 
              0, 
a*8 + b*4 + c*2  + d - P2 == 
              0}, 
{a, b,  c, d}];

v