Step 1: Understanding the Concept:
Marginal Revenue ($MR$) is the rate of change of total revenue with respect to the number of units sold. Mathematically, $MR = \frac{dR}{dx}$.
Step 2: Formula Application:
$R(x) = 3x^2 + 36x + 5$
$MR = \frac{d}{dx}(3x^2 + 36x + 5) = 6x + 36$.
Step 3: Explanation:
Substitute $x = 15$ into the marginal revenue function:
$MR = 6(15) + 36$
$MR = 90 + 36 = 126$.
Step 4: Final Answer:
The marginal revenue is 126.