To simplify the expression -3 x (3/12 + -6/7 + 8/21), we first need to find a common denominator for the fractions inside the parentheses.

The common denominator for 12, 7, and 21 is 84.

Now, let's rewrite the expression with the common denominator:

-3 x (3/12 + -6/7 + 8/21) = -3 x (21/84 + -72/84 + 32/84)

Next, let's add the fractions inside the parentheses:

-3 x (21/84 + -72/84 + 32/84) = -3 x (-19/84)

Finally, let's multiply the fraction by -3:

-3 x (-19/84) = 57/84

Therefore, -3 x (3/12 + -6/7 + 8/21) simplifies to 57/84.