(-3xy+a)(3xy+a)

To simplify the expression \((-3xy + a)(3xy + a)\), we can apply the distributive property (also known as the FOIL method for binomials). This means we will multiply each term in the first parenthesis by each term in the second parenthesis.

Let's do the multiplication step-by-step:

1. Multiply \(-3xy\) by \(3xy\): \[ -3xy \cdot 3xy = -9x^2y^2 \]

2. Multiply \(-3xy\) by \(a\): \[ -3xy \cdot a = -3axy \]

3. Multiply \(a\) by \(3xy\): \[ a \cdot 3xy = 3axy \]

4. Multiply \(a\) by \(a\): \[ a \cdot a = a^2 \]

Now, we combine all these results: \[ -9x^2y^2 - 3axy + 3axy + a^2 \]

Notice that the \(-3axy\) and \(3axy\) terms cancel each other out: \[ -9x^2y^2 + a^2 \]

So, the simplified expression is: \[ \boxed{-9x^2y^2 + a^2} \]