If I have an integral $\int_{A}{u(x_1,x_2,x_3)}$ where $A \subset \mathbb{R}^3$, and I have a function $X:B \subset \mathbb{R}^2 \to A$ with $X(t_1, t_2) = (x_1, x_2, x_3)$ then I can substitute this into the integral to get
$\int_B{u(X(t_1,t_2))|J|}$
where $J$ is the Jacobian matrix but I don't know how to work out what $J$ is. Please advise me. This I find confusing because I am going from $\mathbb{R}^3$ to $\mathbb{R}^2$.
(Suppose here that this is doable, eg., $A$ could be a graph on the domain $B$). Edit: $X(B) = A$ if this makes any difference. So X is a parametrisation.