The nice thing about reflections about perfectly diagonal lines (of gradient 1) is that a line with gradient k will get reflected to become a gradient of 1/k.

The new gradient of the line is 1/2.

The only point on the old line which does not change upon transiting to the new line is the point where the old line intersects the reflection line.

2x + 3 = x + 1
x = -2
y = -1

The new line will also pass the same point (-1, -2).

y = -1/2 x + c
-1 = -1/2 (-2) + c
c = 0

So the equation of the reflected line is y = -1/2 x.

Q16 I look at it some other time if I can still remember.