(a + 9y) • (6a + b);

To simplify the expression \((a + 9y)(6a + b)\), we can use the distributive property (also known as the FOIL method for binomials).

1. Multiply \(a\) by each term in the second binomial:

   • \(a \cdot 6a = 6a^2\)
   • \(a \cdot b = ab\)

2. Multiply \(9y\) by each term in the second binomial:

   • \(9y \cdot 6a = 54ay\)
   • \(9y \cdot b = 9by\)

Now, combine all the terms from these multiplications:

\[ 6a^2 + ab + 54ay + 9by \]

So, the simplified expression is:

\[ 6a^2 + ab + 54ay + 9by \]