How to substituting repeated term

Hi everyone,

Please, I have an equation that has repeated term (x/L) but exponentiate to different numbers. Is there any way to replace these terms by, for example x_L.

My equation:

Y=-((18*x)/L - (45*x^2)/L^2 + (60*x^3)/L^3 - (45*x^4)/L^4 + (18*x^5)/L^5 - (3*x^6)/L^6)

[Michael Haderlein]

You mean like

x_L=x/L;
Y=-18*x_L-45*x_L^2+60*x_L^3-...

?