A lit slit and a screen stand 100 cm apart on an optical bench, as in the picture…

Step 1. The focal length. $f = \dfrac{D^2 - d^2}{4D} = \dfrac{(100\ \text{cm})^2 - (40\ \text{cm})^2}{4 \times 100\ \text{cm}} = \dfrac{8400\ \text{cm}^2}{400\ \text{cm}} = 21.0\ \text{cm}$.

Step 2. The positions. $x_1 = \dfrac{100\ \text{cm} - 40\ \text{cm}}{2} = 30.0\ \text{cm}$ and $x_2 = \dfrac{100\ \text{cm} + 40\ \text{cm}}{2} = 70.0\ \text{cm}$ from the slit.

Step 3. The magnifications. At the first position the object distance is $30.0\ \text{cm}$ and the image distance is $70.0\ \text{cm}$, so $m_1 = \dfrac{70.0\ \text{cm}}{30.0\ \text{cm}} = 2.33$. At the second position they swap, so $m_2 = \dfrac{30.0\ \text{cm}}{70.0\ \text{cm}} = 0.429$.