I looked at a workbook of simple systems of equations exercises. The workbook quality was iffy but I practiced some anyways. I am definitely rusty. I tried tackling 2.57 again but it’s trickier than the workbook stuff (which had e.g. 3 equations of 3 variables all nicely laid out for you).
One issue that I find I have is going in circles proving stuff I already know instead of making progress on the problem.
EDIT: I meant that I had that issue specifically for Exercise 2.57 above.
Finally solved it.
We said x + y = b
We are given b - c = 42°
Because of how exterior angles work, x = y + c, which we can also represent as x - c = y
substitute x + y for b in b - c = 42°
(x + y) - c = 42°
Rearrange the above:
(x - c) + y = 42°
Substitute y for (x - c)
y + y = 42°
simplify
2y = 42°
divide by 2
y = 21°
y is the ∠RQZ they were asking for. 
The part that I was having trouble understanding in their answer basically just says x = y + c, but it’s harder to read so I wasn’t following.
