I have the following equation:

v0=1/b-b/a

Now I have another equation which contains the term v0 but this time I want mathematica to replace v0 with the above value(1/b-b/a). Here is the equation

x0=v0+a+4/b+B/a^2-4B/a

Mathematica does everything right except for replacing the value of v0. It just leaves it as is. Here is the code I am using.

t=4;
xat2=a+4/B+(B*e^-4*a)/a^2
sol2= Solve[x==xat2,x0]

the v0 term is in the equation x, and here is equation x

x0+ ( -1+e^(-t*a)* B)/a^2 + v0t+ B/a*t

I tried using ReplaceAll but that didn’t do anything.Is there something i am missing. I have also attached a png of my code, just in case my equations and explanations are not clear enough.

All i need is for mathematica to replace the v0 with (1/B-B/a) in the last output. Any help would be greatly appreciated.