SA=ph+2b is the right formula for this question.
p= perimeter h= height b=base
Since the base area of this prism is 21cm^2 we can multiply by 2.
21*2=42cm^2
And we know the perimeter of the base is 20 and the height is 8 we just multiply.
20*8= 160cm^2
Now we add
160+42= 202cm^2 is the surface area of this prism.
