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