Can someone just explain this and how to get an answer

The formula we'll use is
s = r*theta

where,
s = arc length
r = radius
theta = angle in radians

In this case,
r = 16 inches
theta = pi/4 radians

So this means,
s = r*theta
s = 16*(pi/4)
s = 4pi

The arc is exactly 4pi inches long.

Since the "pi" part is taken care of already, you just need to enter a whole number. In this case, you would enter 4 into the box.