Answer
$$ y=-\frac{1}{3}x+\frac{5}{3}.$$
Work Step by Step
Since the line is perpendicular to the line given by $ y = 3x + 7$, then its slope is $-\frac{1}{3}$, and hence the line has the equation
$$ y=-\frac{1}{3}x+c.$$
To find $ c $, we make use of the fact that the line passes through $(2,1)$:
$$1=-\frac{1}{3}(2)+c\Longrightarrow c=\frac{5}{3}$$
Hence the equation of the line is
$$ y=-\frac{1}{3}x+\frac{5}{3}.$$
