If the intensity level at distance d of one trombone is 70 db, what is the intensity level of 76 identical trombones, all at distance d?

Since now there are 76 identical trombones, therefore the whole group will be 76 times stronger than if there is only one trombone given that all other things being equal (such as distance and orientation). A dB figure is simply a ratio, in the case of the one trombone, with some reference sound intensity level, such as 10^-12W/m^2. 

To multiply two numbers, add the log of the two numbers.

One trombone is 70 dB, and this is multiplied by a ratio of 76 = 10 * log (76) = 18.8 dB. 

Therefore the resulting sound intensity of the 76 identical trombones is:

70 dB + 18.8 dB = 88.8 dB

88.8 dB