Answer: B
Step-by-step explanation:
To isolate x, you divide both sides by 21
21x=7
[tex]\frac{21x}{21}=\frac{7}{21}[/tex]
x = 7/21
Simplify by dividing both the numerator and denominator by 7
x = 1/3
Since, 1/3 is not an option, B is the correct answer
