Question: resolution of the equation of degree 3

restart:
eq := a*x^3 + b*x^2 + c*x + d;
 s := x1 + x2 + x3 = -b/a;
 sp := x1*x2 + x1*x3 + x2*x3 = c/a;
 p := x1*x2*x3 = -d/a;
sol:=solve({eq, p, s, sp}, {x1, x2, x3},explicit);                       
 a := 1;
b := -5;
c := 6;
d := -1;
 sol;                           

why is not successfull ?   Thank you.                 

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