Let $B$ be a BM. Let $X_t:=2B_t$ and $Y_t:=B_t^2-t$. Compute $\langle X \rangle_t$, $\langle Y \rangle _t$ and $\langle X,Y \rangle _t$.
For the first and second, it's $$\langle X \rangle _t= \langle \int_0^t2dB_s \rangle _t = \int_0^t 4 ds=4t$$ and
$$\langle Y \rangle _t=\langle \int_0^t 2B_s dB_s\rangle_t=\int_0^t4B_s^2ds$$
I'm not sure of what I have to do to get $\langle X,Y \rangle _t $