Explanation

(a) When two lenses of different powers are combined, the power of combination is sum of individual powers
$\therefore P = P₁ + P₂$
$P = 3D + ( - 5D) = - 2D$
Also power = $\frac{1}{f}diopire$
$\therefore f = \frac{100}{P} = \ - \frac{100}{2}m = - 50\ m$
From lens formula
$\frac{1}{f} = \frac{1}{v} - \frac{1}{u}$
Putting $f = \ - 50\ cm,\ u = \ - 50\ cm$
$- \frac{1}{50} = \frac{1}{v} - \left( - \frac{1}{50} \right)$
$\Longrightarrow v = \ - 25\ cm$
Hence, image is formed at same side as object.