#### Answer

$a=-1/21$

#### Work Step by Step

In order to solve this algebraic expression, we must isolate the variable. A variable is the letter in the problem (for instance, in the equation x+2=10, x is the variable). Because addition and subtraction are inverse operations (they cancel each other out), we know we must add when a number is being subtracted from the variable and subtract when a number is being added to the variable.
In the equation 2/7=⅓+a, we must get rid of the 1/3, which is on the same side as a, in order to solve. Thus, we subtract 1/3 on both sides of the equation to find that a=-1/21.