In this question you are finding the surface area. So first you need to classify what is the height length and width.
6 cm- width
4 cm- length
3 cm- height
Next you should draw a rectangular prism. Put the width, length and height in the according sides. After doing so you would see the faces would have the following dimensions: 24, 12 and 18. Multiply each dimension by two and you will get your surface area 108 cm squared.
OR use the following formula: A=2(wl+hl+hw)