How to calculate orbital period with perihelion and aphelion?

By the use of kepers 3rd law which state that the square of the orbital period is proportional to the cube of the average distance from the sun.

that is T^2 is proportional to R^3

write the above as ratio with constant c since the ratio is the same for any body that orbit the sun

by rearranging the expression we get T^2=cR^3 (formula of calculating the orbital period

N. b c is calculated by finding the average of perihen and aphelion