The best advice I can give now is to use a numerical method; e.g. Newton-Raphson, with and .
I'm not sure what the numerical values are, especially
What does ? mean?a=27R^2*Tcr^2?(64*Pcr)
If you don't know how to use Newton-Raphson, work out the numerical values of (don't bother with the units, they only confuse things); post them here, and I'll do it for you.