The equation of the graph at constant temperature for an ideal gas follows Boyle's law, which is:
\[
PV = nRT
\]
Given the graph, it suggests a straight-line relation between pressure and the inverse of volume, which implies that the slope \( m \) of the line is related to the equation. The correct relation would be:
\[
m = \frac{1}{400R}
\]
Thus, the slope is \( \frac{m}{400R} \).