The area of the base of a prism is 21 cm squared. the perimeter of the base is 20 cm. the height of the prism is 8 cm. what is the surface are of the prism? .

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.