Systems, Matrices, and Determinants · Superchallenge problem

Evaluate the block-triangular determinant $\det\begin{bmatrix} 2 & 1 & 5 & 6 \\ 1 & 3 & 7 & 8 \\ 0 & 0 & 1 & 2 \\ 0 & 0 & 3 & 4 \end{bmatrix}$.