To solve this expression, we need to find a common denominator for all the fractions. The least common multiple of 34, 21, and 15 is 510.

Now, we can rewrite the expression with the common denominator:

(15/34) + (7/21) + (19/34) - (20/15) = (15/34) + (7/21) + (19/34) - (32/15)

Next, we need to convert all the fractions to have the same denominator of 510:

(15/34) * (15/15) = 225/510
(7/21) * (24/24) = 168/510
(19/34) * (15/15) = 285/510
(32/15) * (34/34) = 1088/510

Now, we can rewrite the expression with the common denominator:

225/510 + 168/510 + 285/510 - 1088/510

Combining the numerators:

(225 + 168 + 285 - 1088)/510

Simplifying the numerator:

(600 - 1088)/510

Calculating the numerator:

-488/510

Simplifying the fraction:

-244/255

Therefore, 15/34 + 7/21 + 19/34 - 20/15 = -244/255.