2(3a) + (3a) = 9a
Remember the MDAS rule.
In an equation where multiple operations are present, always do these operations in order: Multiplication, Division, Addition, and Subtraction.
2(3a) + (3a) = 0
6a + 3a = 9a
Do not be confused with the second (3a). It denotes multiplication where (3a) = 1(3a)