Answer:
-3 < a < -2
(-3, -2)
Explanation:
Given logarithmic inequality:

For a logarithm to be negative, the base must be between 0 and 1.
Therefore, if the base of the given logarithm is (a + 3), then for the logarithm to be negative:

Solve both inequalities:

Combine the solutions:

Therefore, for logₐ₊₃(7) < 0, the value of a must be greater than -3 and less than -2, represented in interval notation as (-3, -2).
Additional Notes
For a logarithm to be defined, its base should be positive numbers other than 1.