The mass, in kg, of the XXL Pancake at Pancake world is a random variable X whose probability density function is
$$f(x)=\begin{cases}\frac{2x+x^2}{36},&1\leq x\leq 4,\\0,&otherwise.\end{cases}$$
Compute the expected value E[X] and it below as a decimal number accurate to 4 decimal numbers.