Since the initial mass of the substance is 120 milligrams, then:

Therefore, the y-intercept of the graph of the given function is (0,120), also, evaluating the given function at t=0 we get:

Therefore A=120.
Now, we know that after 10 days, the mass of the substance is 90 milligrams, then the graph of the function passes through the point (10,90), also, evaluating the given function at t=10 we get:

Therefore:

Dividing the above equation by 120 we get:

Applying the natural logarithm to the above equation we get:

Dividing the above equation by -10 we get:

Therefore, the function that models this behavior is:

And its graph is:
Answer: