Rectangular prism is a cuboid.
Surface area of a cuboid = 2lw+2wh+2hl
So, here,
Surface area = 2lw+2wh+2hl
=> Surface area = [2(10cm)(3cm)] + [2(3cm)(20cm)] + [2(20cm)(10cm)]
=> Surface area = 60cm² + 120cm² + 400cm²
=> Surface area = 580cm²
