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

The common denominator for 18 and 35 is 18 * 35 = 630.

So, we rewrite the expression with the common denominator:

(-12 * 35)/630 - (-21 * 18)/630

Now, we can simplify the numerators:

(-420)/630 - (-378)/630

Next, we can combine the fractions:

(-420 + 378)/630

Now, we can simplify the numerator:

-42/630

Finally, we can simplify the fraction by dividing both the numerator and denominator by their greatest common factor, which is 42:

-1/15

Therefore, the simplified expression is -1/15.