The top and bottom margins of a poster are 8 cm and the side margins are each 9 cm. If the area of printed material on the poster is fixed at 390 square centimeters, find the dimensions of the poster with the smallest area.

$$\begin{array}{|c|c|c|} \hline & & \\ \hline & printed & \\ & material & \\ \hline & & \\ \hline \end{array}$$

Width = (include units)
Height = (include units)