With the equation:
C = 25( p - 2)
You can substitute/plug in the values of "p" into the equation.
So when p is 6, you can plug in 6 for p in the equation.
C = 25(p - 2)
C = 25(6 - 2)
C = 25(4)
C = 100
And you continue to plug in values for p.
I'm not sure how you are suppose to write the equation into "RULE". You could write 25(6-2) or 25(4), unless they have already showed you how you are suppose to write it