If you're asked to find one quantity A
as a function of some other quantity x,
then you should set up a system of equations
with one more variable than equations.
You won't be able to solve this completely
and find out what A is definitively,
but that's OK;
all that you need to do
is to solve for A in an equation that still has x in it.
The general strategy is to solve any equation with x in it
for any variable other than x itself
and then substitute that solution into the other equations.
Eventually, this should leave you
with an equation with only A and x in it;
when you solve that equation for A, then you're done.
This is all much clearer with examples,
and while there are some examples in the book,
they may make the whole thing seem harder than it has to be.
The book generally makes do with as few equations and variables as possible,
which has the advantage of keeping the thing short,
but at the disadvantage of making each equation hard to find.
So I've done an example where there are many equations and many variables
but each individual equation is very simple and easy to think of.
Check out the video: