Mei Lin cannot see clearly any object that is farther than 80 cm from her eyes…

Step 1. The focal length. $u = -\infty$ and $v = -80\ \text{cm}$. So $\dfrac{1}{f} = \dfrac{1}{v} - \dfrac{1}{u} = -\dfrac{1}{80\ \text{cm}}$ and $f = -80\ \text{cm}$.

Step 2. The power. $P = \dfrac{1}{f} = \dfrac{1}{-0.80\ \text{m}} = -1.25\ \text{D}$.