Answer
$a=-\frac{1}{21}$
Work Step by Step
First we need to subtract $\frac{1}{3}$ from both sides to isolate the variable:
$\frac{2}{7}-\frac{1}{3}=\frac{1}{3}-\frac{1}{3}+a$
In order to go further, we must find a common denominator to subtract the fractions. The quickest way to find a common denominator is to multiply the two denominators together; $7*3=21$:
$\frac{2}{7}*\frac{3}{3}=\frac{6}{21}$ and $\frac{1}{3}*\frac{7}{7}=\frac{7}{21}$
Now we can substitute those new fractions in our equation so we can subtract them
$\frac{6}{21}-\frac{7}{21}=a$
When subtracting fractions, you subtract the numerators but you keep the denominator the same
$-\frac{1}{21}=a$
