Answer
Makes sense.
Work Step by Step
Yes, you could have simplified the parentheses by combining like terms first:
$3(2x+5x)=3(7x)=(3\cdot 7)x=21x$
(using the associative property instead).
Makes sense.
Note:
Although "combining like terms mentally" actually does involve applying the distributive property,
the idea here is that the 3 was not distributed as $3(2x)+3(5x).$
