A convex lens forms a real image that is 4 times as large as the object…

Step 1. The image. $v = m \times u = (-4) \times (-15\ \text{cm}) = +60\ \text{cm}$.

Step 2. The focal length. $\dfrac{1}{f} = \dfrac{1}{60\ \text{cm}} + \dfrac{1}{15\ \text{cm}} = \dfrac{1 + 4}{60\ \text{cm}} = \dfrac{1}{12\ \text{cm}}$, so $f = 12\ \text{cm}$.