To find the diagonals we use the pythagorean theorem:

in each case the diagonal is the hypotenuse of the triangle.
For the base diagonal we have that the legs of the triangle are l and w, then we have:
![\begin{gathered} c^2=5.1^2+8.5^2 \\ c=\sqrt[]{5.1^2+8.5^2} \\ c=9.9 \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/college/nx69xrjgtozs8z8eqaexoskeyv6fh7vs60.png)
Therefore the base diagonal is 9.9
Now, the second diagonal is the hypotenuse of the triangle with legs h and base diagonal, then we have that:
![\begin{gathered} d^2=9.9^2+5.8^2 \\ d=\sqrt[]{9.9^2+5.8^2} \\ d=11.5 \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/college/vx8khprfarg4rdw9pb907dxfhhlu9kj6rp.png)
Therefore the second diagonal is 11.5