Answer
a) $9I_1$
b) $I_1$
Work Step by Step
$$\color{blue}{\bf [a]}$$
The author told us that the path-length difference is $\lambda$ which means that the interference is constructive (since we assume that the light is a monochromatic light beam).
Thus,
$$I_{max}=N^2I$$
where $N$ is the number of waves interfered (or the number of slits)
$$I_{max}=3^2I_1$$
$$\boxed{I_{max}=9I_1}$$
$$\color{blue}{\bf [b]}$$
We have the same three slits with the seam monochromatic light and when the path length difference between any two adjacent slits is $\lambda/2$, there will be a perfect destructive interference between the first two slits, and then a full $\lambda$ remains between the first and the third slit.
Thus,
$$\boxed{I=I_1}$$
