Answer
Write the distribution as the sum of two products, and then simplify:
$3(5x+7)=3\cdot 5x+3\cdot 7$
and then simplify to
$ 15x+21$
Work Step by Step
The student forgot to distribute the 3 to the second term in the parentheses.
To avoid this error, maybe it's best to write the distribution as the sum of two products, and then simplify:
$3(5x+7)=3\cdot 5x+3\cdot 7$
and then simplify to
$ 15x+21$
