You can't set p to a value that depends on v, which is uninitialized, so what will happen is that the value that was input for p will be overwritten by a garbage value.

You need to do some algebra first. You have
[tex]p~=~\frac{r~t}{v - b} - \frac{a}{v^2}[/tex]

which I assume is the correct equation.

If you multiply both sides of this equation by v^{2}(v - b), you get this equation:
[tex]p~v^2(v - b)~=~r~t~v^2 - a(v - b)[/tex]

Simplifying and moving terms around results in the following equations:

Nor can you use C++, C, C#, Fortran, Basic, and most other programming languages to solve quadratic or even linear equation. They aren't programs; they are general purpose programming languages that you can use to write programs.