how to substitute several variables on both sides of the equation?

edit

This is my worksheet: https://cocalc.com/share/f7766c5e-2f4...

I would like to substitute

V == 4*pi*p_ges/U

in line 36 in the equation

 SR.symbol('p_ges') == p_ges_rhs

but those variables are spread out over both sides of the equation, and i am clueless as to how to convince sage that this is a good idea. :-) How can I transform this equation in this way? Do i need to do this manually and re-write p_1 -p_4 accordingly?