Start with expression [tex]ax+b-4=b.[/tex]
1. Separate terms - with x leave in left side, without x write in right side:
[tex]ax=b-b+4,\\ \\ax=4.[/tex]
2. Divide the last expression by a ([tex]a\neq 0[/tex]):
[tex]x=\dfrac{4}{a}.[/tex]
Answer: this statement is true when [tex]a\neq 0[/tex].
