A convex lens forms a real image of the same size as the object, on a screen 56 cm from the lens.

Step 1. The object. $m = \dfrac{v}{u} = -1$, so $u = -56\ \text{cm}$.

Step 2. The focal length. $\dfrac{1}{f} = \dfrac{1}{56\ \text{cm}} + \dfrac{1}{56\ \text{cm}} = \dfrac{1}{28\ \text{cm}}$, so $f = 28\ \text{cm}$.

Step 3. The power. $P = \dfrac{1}{0.28\ \text{m}} = +3.57\ \text{D}$.