In this case, we have a rectangular prism with three pairs of equal faces, equal faces are located on opposite sides like the front and back faces or top and bottom faces. Then in order to calculate the total surface area of the prism we just have to find the area of each different face, multiply it by 2 so we take into account its equal pair and then sum them all up, like this:
Total surface area = 2×(7×5) + 2×(3×5) + 2×(7×3) = 2×35 + 2×15 + 2×21 = 70 + 30 + 42 = 142
Then, the total surface area of the given prism is 142 cm²