Answer
$$C = 0$$
Work Step by Step
$$\eqalign{
& F\left( x \right) = {x^2} - 3x + C,{\text{ we know that }}F\left( { - 1} \right) = 4,{\text{ then}} \cr
& 4 = {\left( { - 1} \right)^2} - 3\left( { - 1} \right) + C \cr
& 4 = 1 + 3 + C \cr
& 4 = 4 + C \cr
& C = 0 \cr} $$