Final answer:
To simplify the expression 3(6x + x), distribute the 3 to get 18x + 3x, which simplifies to 21x, corresponding to option (a).
Step-by-step explanation:
The student is asking to simplify the expression 3(6x + x). To simplify this expression, we distribute the 3 across the terms inside the parentheses:
3(6x) + 3(x) = 18x + 3x
Now, we combine like terms:
18x + 3x = 21x
Therefore, the simplified form of the expression 3(6x + x) is 21x, which corresponds to option (a).