There are times when simple algebraic operations are confused by introducing trigonometric functions or logarithmic terms.
13 = 7 - 3x
This can be easily computed to be
13 - 7 = - 3x
==> 6 = - 3x
==> x = -2
But how about 13 = 7 - 3 tan X ?
Solution: 13 = 4 tan X ==> tan X = 13 / 4 , ..... and got into hot soup!
A careless mistake has been made.
When tan X was substituted into the original equation, the eyes refused to acknowledge this "complicated" tan X.
The eyes can only see the simpler "7 - 3" and thus compute it to be (7 - 3) = 4!
This caused the 7 - 3 tan X to be 4 tan X, which is WRONG.
The correct mathematical process of solving should maintain.
13 = 7 - 3 tan X
==> 13 - 7 = - 3 tan X
==> 6 = - 3 tan X
==> tan X = -2
Maths is not that complicated when you follow the rules closely, even when the terms have changed into a seemingly complex expression / term.
By following what you have known with simple expression / term, any challenging equation can be easily solved.
This is the power of learning maths properly.
Being discipline in the way you handle maths is the key.
With a discipline mind, maths becomes fun , .. and interesting.