.
The way on how the other tutor resolved this problem/question, is formally correct,
but this problem can be solved in MUCH EASIER way.
What is even more important, this problem IS DESIGNED and IS INTENDED to be solved by the other, more simple way.
See my solution below.
The identity (A + B) - A = B directly instructs and prescribes you to use the formula of difference for tangent function,
which originally has the form
tan(x - y) =
Using this formula, you should take x = A+B, y = A to get
tan(B) = tan((A+B) -A) = =
now substitute the given values tan(A+B) = and tab(B) = 3 to get
= = = = = -2. ANSWER
In this way, the problem is just SOLVED.
Notice, that this solution DOES NOT REQUIRE reducing the problem to some equation and solving it.
It only requires to make one line simple calculations based on well known Trigonometry formula.
If my solution seems complicated to you from the first glance (although it is as simple as possible),
I will repeat it one more time, by omitting accompanying explanations - they are needed only for the first time:
tan(B) = tan((A+B) -A) = = = = = = -2.
This one line version is how the solution should be and must be presented when you know the way on how to solve it . . .
Once again: my post is the way on how this problem expected and intended to be solved.
The other's tutor solution is a wrong approach to the problem.