To simplify the expression, we can factor out sin^2 a from the numerator and denominator:

(9sin^2 a) / (3 - 3sin^2 a)

Now we can factor out 3 from the denominator:

(3sin^2 a) / (3(1 - sin^2 a))

Recognizing that sin^2 a is equal to 1 - cos^2 a:

(3sin^2 a) / (3(cos^2 a))

Now we can simplify it further by canceling out the 3 in the numerator and denominator:

sin^2 a / cos^2 a

Using the trigonometric identity tan^2 x = sin^2 x / cos^2 x, we can rewrite the expression as:

tan^2 a