I'm looking for some help in functional differentiation. I am self-studying Lancaster & Blundell, and the question below came up. I took a look at the solutions but they were confusing to me and skipped steps. Any help would be appreciated!
Find the functional derivative of $$\dfrac{\delta^2 I[f^3]}{\delta f(x_0)\delta f(x_1)}$$
Where $$I[f]=\int^1_{-1} f(x) dx$$
Thank you in advance!