Hint: Use the mirror formula.

Step 1: Find the virtual object distance for the secondary mirror.
A Cassegrain telescope consists of a concave and a convex mirror.

Distance between the objective mirror and the secondary mirror, d = 20 mm

The focal length of the objective mirror, f₁=$-\frac{{\mathrm{R}}_1}{2}=-110 \mathrm{mm}$

The focal length of the secondary mirror, f₂=$-\frac{{\mathrm{R}}_2}{2}=-\frac{140}{2}=-70 \mathrm{mm}$​​

The image formed by the objective lies at its focus.

The image of an object placed at infinity, formed by the objective mirror, will act as a virtual object for the secondary mirror.

Hence, the virtual object distance for the secondary mirror, u = f₁+d =- 110+20 = -90 mm

Step 2: Find the position of the final image.
Applying the mirror formula for the secondary mirror,

$\frac{1}{\mathrm{f}}=\frac{1}{\mathrm{v}}+\frac{1}{\mathrm{u}}$
$-\frac{1}{70}=\frac{1}{\mathrm{v}}-\frac{1}{90}$
$\frac{1}{\mathrm{v}}=\frac{1}{90}-\frac{1}{70}=\frac{7-9}{630}=-\frac{1}{315}$
$\mathrm{v}=-315 \mathrm{m}$

Hence, the final image will be formed 315 mm away from the secondary mirror.