To simplify this expression, we need to find a common denominator for the fractions:

-1/21 + 3/14

To find a common denominator, we need to find the least common multiple of 21 and 14, which is 42.

Now, we can rewrite the fractions with the common denominator of 42:

-2/42 + 9/42

Now that the fractions have a common denominator, we can combine them:

-2/42 + 9/42 = 7/42

Therefore, the simplified expression is:

X = 7/42