Answer: 5/36
First, find the LCD of both the denominators (in this case, 9 and 4).
LCD = 36
Then, multiply each fraction by the number that would give you a denominator of the LCD that you found earlier (in this case, 36).
(8/9) · 4 + (-3/4) · 9 = (32/36) + (-27/36)
Now, you just have to add the numerators because the fractions have the same denominator.
(32/36) + (-27/36) = 5/36
∴ it equals 5/36.