Analysis: This question examines the application of linear equations with one variable. According to the topic, Xiao Gang did the X problem correctly. Through the known conditions of the topic, he listed the equation and got the result.

The problem solving process is as follows:

Solution: Let 2 1 13 only do the X problem correctly.

9x-3( 12-x)=84

9x-36+3x=84

12x=84+36

12x= 120

x= 10

Vertical types are as follows:

The wrong question is: 12- 10=2 (Tao)

A: Wang Gang made two mistakes.

Extended data:

Root seeking method

conventional process

There are five steps to solve a linear equation with one variable, namely, denominating, removing brackets, shifting terms, merging similar terms, and the transformation coefficient is 1. All steps are carried out according to the properties of algebraic expressions and equations.

In one-dimensional linear equations, the denominator removal step is usually multiplied by the least common multiple of each denominator. If the denominator is a fraction, it can be replaced by the other part of the item multiplied by the reciprocal of the fraction on the denominator.

If there is an irrational number on the denominator, it is necessary to make the denominator rational first.