Answer
A scatterplot can be used to find the equation of the trend line by using two points on the scatterplot to find the slope of the trend line (and then replacing the $m$ with the slope in the formula of a line).
The equation of the trend line would be used to make the estimate by replacing either $x$ or $y$ (where applicable) in the formula of a line (given the slope, which can be found with two points on the scatterplot.
diameter = 7 cm
Work Step by Step
$circumference = diameter * \pi$
$C = \pi * d$
$22 = \pi * d$
$22/$$\pi$ = $\pi$ * d / $\pi$
$22/3.14 = d$
$7 = d$
