Step 1:

Since the marginal cost is the rate of change of total cost w.r.t the output.

Therefore marginal cost (MC)=$\large\frac{dc}{dx}$$(x)$

$\qquad\qquad\qquad\qquad\qquad=\large\frac{d}{dx}$$(0.007x^3-0.003x^2+15x+4000).$

$\qquad\qquad\qquad\qquad\qquad=0.007\times 3x^2-0.003\times 2x+15$

Step 2:

When $x=17$ we have,

$\qquad\qquad\qquad\qquad\qquad=0.007\times 3(17)^2-0.003\times 2(17)+15$

$\qquad\qquad\qquad\qquad\qquad=6.069-0.102+15$

$\qquad\qquad\qquad\qquad\qquad=Rs 20.967$.