Step 1:

In the orbit of mercury,$e=0.206$ and semi-major axis $a=36$(in millions of miles).

Let $F(ae,0),F'(-ae,0)$ be the foci and $A(a,0),A'(-a,0)$ be the vertices of the elliptical path with the sun at $F$.

Step 2:

Mercury is closest to the sun at $A$.

Distance from the sun =$FA=a(1-e)$

$\qquad\qquad\qquad\qquad=36(1-0.206)$

$\qquad\qquad\qquad\qquad=36\times 0.794$

$\qquad\qquad\qquad\qquad=28.584$ million miles.