A small tent frame is a tetrahedron with corners A(2, 0, 1), B(-1, 4, 5), C(3, 2, -3) and a fourth…

Step 1. The sums. The four $x$-values add up to $4 \times 1 = 4$. The three known ones add up to $2 - 1 + 3 = 4$. So $x_D = 0$.

Step 2. The $y$-values. $4 \times 2 = 8$ and $0 + 4 + 2 = 6$. So $y_D = 2$.

Step 3. The $z$-values. $4 \times 2 = 8$ and $1 + 5 - 3 = 3$. So $z_D = 5$.