Step-by-step explanation:
[tex]\bullet\hookrightarrow\sf 7 (x+4)=21 [/tex]
[tex]\bullet\Rrightarrow\sf 7x+28=21 [/tex]
[tex]\bullet\rightarrowtail\sf 7x=21-28 [/tex]
[tex]\bullet\twoheadrightarrow\sf 7x=-7 [/tex]
[tex]\bullet\looparrowright\sf x=\dfrac{-7}{7}[/tex]
[tex]\bullet\rightharpoonup\sf x=-1 [/tex]
