The cross-section of a mirror is the curve 4x² + y² = 16, with lengths in decimetres…

Step 1. Standard form. $\dfrac{x^2}{4} + \dfrac{y^2}{16} = 1$. The larger denominator is under $y^2$, so $a = 4$ and $b = 2$.

Step 2. The focus. $c = \sqrt{16 - 4} = 2\sqrt{3} \approx 3.46$ decimetres. The foci are $(0, \pm 3.46)$.

Step 3. The eccentricity. $e = \dfrac{c}{a} = \dfrac{2\sqrt{3}}{4} = \dfrac{\sqrt{3}}{2} \approx 0.866$.

Step 4. The latus rectum. $\dfrac{2b^2}{a} = \dfrac{2 \times 4}{4} = 2$ decimetres.