Step 1:

Since the marginal revenue is the rate of change of total revenue w.r.t the number of units sold.

Marginal revenue (MR)=$\large\frac{dR}{dx}$

$\qquad\qquad\qquad\qquad=\large\frac{d}{dx}$$\big(13x^2+26x+15)$

Step 2:

On differentiating we get,

$\large\frac{dR}{dx}$$=26x+26$

The marginal revenue when $x=7$

$\large\frac{dR}{dx}=$$26\times 7+26$

$\qquad=Rs.208$