Straight through the curves

Let $V = P_{n} \left(\mathbb{R}\right)$ be an inner product space of all polynomials with real coefficients and degree not greater than $n$ , and the inner product defined as $\langle f(x) , g(x)\rangle = \int_{-1}^{1}f(t)g(t)dt \: \forall f,g \in V$ The orthogonal projection of $f \in P_{3}\left(\mathbb{R}\right)$ , $f(x)=x^3$ on $P_{2}\left(\mathbb{R}\right)$ is a line. Find the slope of this line.