Write the formula for the surface area of a rectangular prism​

F(x)=x/2-2 and g(x)=2x^2+x-3 find (f+g)(x)

Could someone me with this question i’ve been stuck on it for 20 minutes

The perimeter of ? cde is 55 cm. a rhombus dmfn is inscribed in this triangle so that vertices m, f, and n lie on the sides cd , ce , and de respectively. find cd and de if cf=8 cm and ef=12 cm. the answer is not cd=20 and de=15. i have tried that one, it does not work.