If P(−1, 1) is the midpoint of the line segment joining

(b) −1
The given points are A(−3, b) and B(1, b+4).

Then, $\left(x_{1}=-3, y_{1}=b\right)$ and $\left(x_{2}=1, y_{2}=b+4\right)$

Therefore,

$x=\frac{[(-3)+1]}{2}$

$=\frac{-2}{2}$

$=-1$

and

$y=\frac{[b+(b+4)]}{2}$

$=\frac{2 b+4}{2}$

$=b+2$

But the midpoint is $P(-1,1)$.

Therefore,

$b+2=1$

$\Rightarrow b=-1$