To solve the equation, you must isolate s, which means to get it alone on one side. Since you are multiplying s by 21, you must divide by 21 to get s alone. However, whatever you do to one side of the equation you must do to the other, so if you divide both sides by 21 and simplify it will give you s.
[tex]21s=3 \\ \\ s= \frac{3}{21} \\ \\ s= \frac{1}{7} [/tex]
