The two triangular ends have area (3 cm)*(4 cm) = 12 cm^2, so the area of the rectangular faces is (204 -12) cm^2 = 192 cm^2. The sum of lengths of those rectangular faces is (3 cm +4 cm +5 cm) = 12 cm, so the width is given by
.. 192 cm^2 = (12 cm)*x
.. (192 cm^2)/(12 cm) = 16 cm = x
The 2nd selection is appropriate.
