Your question is not well formulated.
Sorry, let me rewrite the question exactly as it was given in my question paper :
"Let P(subscript 11) denote the set of all polynomials of degree < = 11. Let S denote the subspace of all polynomials , f , in P(subscript 11) satisfying f(10)=f(11)=f(12). Compute the dimension of S, giving clear explanation."