Marginal Analysis of Office Complex

Problem: 

The cost and revenue functions for an office complex are R(r)=0.004r2+55.4r and C(r)=42,30012r, respectively, where r is the monthly rent. Determine the marginal revenue and cost for monthly rent of $3,000 and $8,000.

Answer: 

$31.40 and $8.60 for rents of $3,000 and $8,000, respectively.