Solve Symbolic Simultaneous equations -- Please help!!

I am attempting the following code to solve for a,b,c, and d. i keep getting an error saying a does not exist, what i am looking for is a solution for a b c d in terms of n and t

clear all; close all; clc;
syms a b c n t d
eqns = [ a + b + c + d == 0, a*(n^3)+ b*(n^2)+ c*n+ d == t, 3*a*(n^2)+ 2*b*n +c == 0];
S = solve(eqns);

S.a
S.b
S.c
S.d

the error i get is

Reference to non-existent field 'a'.

Error in Untitled (line 7)

S.a

[Alan Stevens]

You are asking for four unknowns but you only have three equations!

Answer 1

[Manvi Goel]

You will require four equations to calculate values of four unknowns. You only have 3 here.

a + b + c + d == 0, (1)

a*(n^3)+ b*(n^2)+ c*n+ d == t, (2)

3*a*(n^2)+ 2*b*n +c == 0, (3)

Answer 2

[Walter Roberson]

S = solve(eqns, [a, b, c, d]);