(а + 3) (а - 6)

To simplify the expression \( (a + 3)(a - 6) \), you can use the distributive property (also known as the FOIL method for binomials).

Here's how it's done step-by-step:

1. First: Multiply the first terms: \( a \cdot a = a^2 \).
2. Outer: Multiply the outer terms: \( a \cdot (-6) = -6a \).
3. Inner: Multiply the inner terms: \( 3 \cdot a = 3a \).
4. Last: Multiply the last terms: \( 3 \cdot (-6) = -18 \).

Now, combine all those results together:

\[ a^2 - 6a + 3a - 18 \]

Combine the like terms:

\[ a^2 - 3a - 18 \]

So, the simplified expression is:

\[ a^2 - 3a - 18 \]