Here is my problem: Let Y=Z-3X^2 where X and Z are normally distributed N(0,1). Simulate n=50 observations of (X,Y) and plot Y against X. Estimate ρ(X, Y) and comment the result. What is the true ρ in this case? Does the result indicate that X and Y are stochastic independent?

I have done the plot and has had Excel estimate r=-0,1416, but I cannot understand how to calculate the true value of ρ. Maybe someone can give me a hint?