To solve P = (R*T)/(V-b) - a/(V*(V+b)*sqrt(T)) for V when everything other than V is known, you can subtract the left hand side from both sides, giving
P-P = (R*T)/(V-b) - a/(V*(V+b)*sqrt(T)) - P
or
0 = (R*T)/(V-b) - a/(V*(V+b)*sqrt(T)) - P
now you have an equation that should be equal to 0, and you can do a bisection search for the V that make (R*T)/(V-b) - a/(V*(V+b)*sqrt(T)) - P equal to 0. You don't need to do much rearranging at all.
Hint: consider using anonymous functions.
If you have the symbolic toolbox then you could solve the equation with V and T symbolic in order to get a formula for V in terms of T, but at some point you would still need to substitute in a T in order to get the answer to the assignment.